Struggling to understand QMHSNDPM API

Hello,

I want to show messages from a message file in the message line of a DSPF with ERRSFL set. I wrote a standalone program with a DSPF to learn how to accomplish that. You can download and look here:
- https://leela.pocnet.net/~poc/msglindf.txt
- https://leela.pocnet.net/~poc/msglinpg.txt

Additionally, there's a message file QGPL/GENERICMSG (created by me) with a message HLO0001 just containing a hello for first- and second level text and *NONE for any answering parameters. (I'm using this very message file to successfully show messages with ERRMSGID and ERRSFLMSGID in some DSPFs.)

Before anyone is wondering: I'm using an old box with V4R5 for tinkering, so I can't use free or all-free. Reasons for that are out of scope for my question. :-)

I read sources I found via Google:
- https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_74/apis/QMHSNDPM.htm
- https://www.mcpressonline.com/programming/rpg/sending-a-program-message-in-rpg
- http://newsolutions.de/forum-systemi-as400-i5-iseries/threads/1716-QMHSNDPM

They are somewhat deviant in how the prototype variables are defined.

After the first press of F10, there's no change in display, especially, there is no message shown. After the second F10, the *PGM is ended with RNX9998. I'm stuck here, since I can't find helpful documentation for RNX9998, nor where I could be wrong with my API call.

May I kindly ask for advice?

:wq! PoC

PGP-Key: DDD3 4ABF 6413 38DE - https://www.pocnet.net/poc-key.asc