You need to first disable host printing and enable raw print mode in ProComm. Then go to maintenance, issues your formprint command, i.e.

formprint multiline set assignment /port lpr3

Then you need to issue an enable print command.

en pr

At that point you will see your formprint scroll the screen. Either have a capture file set up or print capture (or have a large enough scroll buffer in Procomm). After you see formprint complete message, issue a Ctrl-C to get back to maintenance mode.