Sage 200 CRM Card Payment Form Launch

I'm trying to create a button in CRM to launch the 'Online Card Receipt' form (Sage.MMS.SalesLedger.OnlineCardReceiptForm). I think I know how to add the button, but I am not sure how to launch the form since it doesn't use a standard form ID as described here

This post talks about adding additional form IDs for other forms using an XML file but doesn't go into detail about how to use the XML file or what to use for the objectType or assembly variables. 

Is anyone able to explain how I can use this to launch the Online Card Receipt form?