Instead of trying to launch the panel, you might be better off invoking the SO_CancelReason lookup instead. I'm not sure if this will actually work in a UDS as long as you have a handle to the UI object, such as oSession.AsObject(oScript.UIObj).InvokeLookup but if it does, you will want to make sure you are actually filtering for very specific details, such as the UI being present in the first place, can use oScript.UIObj <> 0, the start program, might want to make sure it is SO_SalesOrder_UI, make sure oSession.Updating = 0, etc.

Another way to do this might be to use oSession.AsObject(oScript.UIObj).NomadsProcess "DCANCEL", "SO_SalesOrder.M4L" but you may need either a UI post exit script or a fake OK button to run a button script to capture the value entered in the field so you can set it into a storage variable using oSession.AsObject(oSession.ScriptObject).SetStorageVar. You would then use oSession.AsObject(oSession.ScriptObject).GetStorageVar in your original script to get the value that was entered. You could then loop if the value returned is blank in case the user just tried to click OK or Cancel. 

I would try the InvokeLookup first though as it will be much cleaner and efficient. The first argument is the lookup name, i believe "SO_CancelReason" should give you what you want. The second argument is the variable you want the select value returned in. Make sure you initialize the variable as a blank string before you pass it to the method.

Awesome, thanks for the great info as always David Speck.  I'm going to take it and run with it.  If I stumble (I'm sure I will), I'll post a reply.  

My concern would be that anything set in a script during the Pre-Delete event might not make it into SO History.

Good point, may actually have to move to a post delete and attempt to update the history record afterwards. I haven't messed with history and pre deletes much.

I'd almost lean towards a custom button, invoking BT_ACCEPT first (prompting then setting the reason code value), then re-opening the SO to process the delete.

That's what I'm seeing.  I can bring up the lookup, select it, and it looks like it deletes ok.  However, it does not flow into SO_SalesOrderHistory as a pre-delete script.  I will try David's suggestion as a post-delete and write to history.

You could use a fake Delete button, hide the real one, have the fake delete button prompt with the same verbiage confirming they want to delete the order, if they answer No, do nothing, if they answer Yes, then proceed to invoke the lookup, get the value back, set it into the cancel reason field, either use oBusObj.Write or oUIObj.BT_Accept/oScript.InvokeButton "BT_Accept", then then call oBusObj.Delete and pass it the sales order number, may then need to follow up with oUIObj.BT_Cancel/oScript.InvokeButton "BT_Cancel".

If you use the InvokeButton methods, you may also need the oUIObj.HandleScriptUI immediately after each InvokeButton. You can check the return value when doing the "accept" step to see if the order was saved successfully or if an error or warning occurred. The only reason the final step for BT_Cancel might be needed is if the panel doesn't reset to the default state where the fields are empty after you use the finish the "accept" step. If you use oUIObj.BT_Cancel, you may need to follow up after it immediately with oUIObj.RefreshScrn

Hi David,
Thank you again for your excellent advice.  I am currently working on the fake button method (slowly but surely, one step at a time since I'm still a newbie at this) and I was able to create a fake button, which once pressed gives me the two options.  When clicking Yes, I invoke the lookup and catch the value, which I then try to pass onto the order.  However, it is not writing it back to the order for some reason.  Is it because by invoking the lookup it loses the handle to the SO_SalesOrder_bus?  I thought so, so I made sure to call it back.  However, it still doesn't work.   Here is my code (a work in progress)

r=oScript.DebugPrint("Deleting Sales Order with Custom Delete button")
sMsg = "StartProgram = " & oSession.StartProgram & " Updating = " & CStr(oSession.Updating)
r=oScript.DebugPrint(sMsg)
oUIObj = 0
response = ""
oSOWrite = ""
oSOCancel = ""

If IsObject(oUIObj) = False Then
oUIObj = oScript.UIObj
Set oUIObj = oSession.AsObject(oUIObj)
End if

If oScript.UIObj <> 0 and oSession.Updating = 0 then

SONo = ""
retVal = oBusObj.GetValue("SalesOrderNo$",SONo)
r=oScript.DebugPrint("SONo = " & SONo)
sMsg = ("Do you want to delete Sales Order " & SONo)
r=oScript.DebugPrint(sMsg)
response = oSession.AsObject(oSession.UI).MessageBox("",sMsg,"style=YesNo,icon=?")

If response = "YES" Then
sMsg = ("Deleting SO " & SONo)
r=oScript.DebugPrint(sMsg)

retVal = oUIObj.InvokeLookup("SO_CancelReason",oSOCancel)
sMsg = ("Cancel Code = " & oSOCancel)
r=oScript.DebugPrint(sMsg)
set oSOWrite = oSession.AsObject(oSession.GetObject("SO_SalesOrder_bus"))
retVal = oSOWrite.SetKeyValue("SalesOrderNo$",SONo)
retVal = oSOWrite.SetKey()
retVal = oSOWrite.SetValue("CancelReasonCode$",oSOCancel)
retVal = oSOWrite.Write()
retVal = oSOWrite.Clear()

oUIObj.BT_Accept

Else
sMsg = ("Not deleting SO " & SONo)
r=oScript.DebugPrint("sMsg = " & sMsg)
End If

End If

I would do something more along the lines of this. 

sMsg = "Deleting Sales Order with Custom Delete button"
oScript.DebugPrint sMsg
sMsg = "StartProgram = " & oSession.StartProgram & " Updating = " & CStr(oSession.Updating)
oScript.DebugPrint sMsg

If oScript.UIObj > 0 And oSession.Updating = 0 Then
	If Not(IsObject(oUIObj)) Then Set oUIObj = oSession.AsObject(oScript.UIObj)
	sSalesOrderNo = "" : oBusObj.GetValue "SalesOrderNo$", sSalesOrderNo
	oScript.DebugPrint "sSalesOrderNo: " & sSalesOrderNo
	sMsg = "Do you want to delete Sales Order " & sSalesOrderNo
	oScript.DebugPrint sMsg
	sResponse = "" : sResponse = oSession.AsObject(oSession.UI).MessageBox("", sMsg, "style=YesNo,icon=?")
	If UCase(sResponse) = UCase("Yes") Then
		sMsg = "Deleting SO " & sSalesOrderNo
		oScript.DebugPrint sMsg
		sCancelReasonCode = "" : oUIObj.InvokeLookup "SO_CancelReason", sCancelReasonCode
		oBusObj.SetValue "CancelReasonCode$", sCancelReasonCode
		If oUIObj.BT_Accept() <> 0 Then
			oBusObj.Delete sSalesOrderNo
			oUIObj.BT_Cancel
			oUIObj.RefreshScrn
		End If
	Else
		sMsg = "Not deleting SO " & sSalesOrderNo
		oScript.DebugPrint sMsg
	End If
End If

A button script can already access the oBusObj so there is no need to get another handle as you are currently doing, more than likely, your current SetKey is failing because the sales order is still in use by the current oBusObj. 

I also initialize my variables on the same line they are used to be sure that they are initialized and not forgotten, which can happen if you always put them at the beginning. 

VBScript has a weird nuance when using parenthesis with a method when you aren't returning the result into a variable, think this happens either when the method has only one argument or more than one argument, been a while since i've seen it because i stopped using the parenthesis when they weren't needed. I only use them if i'm returning the value into a variable or if i need to evaluate the returned value.

Thanks so much for this, David.  Lots of good stuff here for me to learn.

I tried the script and unfortunately it still does not write to the SO Header, which then does not write to history.  Also, I noticed it creates a blank records in SO_SalesOrderHistoryHeader table.  I tried adding oBusObj.Write() after the SetValue but that didn't do anything either :(