You would need to get an object handle to the PO_PurchaseOrder_Bus object.  Set the header field values, then use its Lines property to access the detail object to add lines and set their values, use the Write method when done with each line and then finally use the header's Write method to save it.

You can search the forum for examples of header/line objects.  Here are two threads to get you started.

(+) Creating PO via BOI Write Issue - Business Object Interface - Sage 100 - Sage City Community

(+) P.O. Integration - Business Object Interface - Sage 100 - Sage City Community