It depends on how many form parts you have.

Here is an example sends XML data in one part and another xml file in a second part:

// xml meta  data

lblob_data = blob ("--"  + ls_BOUNDARY + "~r~n" + &
                 'Content-Disposition: form-data; name="meta"' + &
                 "~r~n~r~n" + ls_xml + "~r~n", EncodingUTF8!)

// a file
lblob_data += blob ("--"  + ls_BOUNDARY + "~r~n" + &
                 'Content-Disposition: form-data; name="file"; filename="stammdaten.xml"' + &
                 "~r~nContent-Type: application/xml~r~n~r~n", EncodingUTF8!) + &
                 ablob_xml + blob ("~r~n--"  + ls_BOUNDARY + "--~r~n", EncodingUTF8!)

The boundary string (ls_boundary) you have to specify in the request header. It is to separate the parts of the message. So it must not be a string that is in one of the message parts.