SET %field1 = "field1 value"
SET %field2 = "field2 value"
SET %filename = "C:\files\files.zip" 

FTPLOGON "HTTP Site"
PREPAREPOST /contenttype="multipart/form-data"
;; you may be able to use /contenttype="application/x-www-form-urlencoded" if you are not uploading a file
postvalue "field1" %field1
postvalue "field2" %field2
postvalue "userfile" %filename /file /contenttype="application/zip"
;; Leave above line out if you are not posting a file
HTTPPOST "/poster.asp" "" %postresult /outtype=string /intype=prepared 
FTPLOGOFF