On 09 Feb 2020, metalhead said the following...

I apologize if it's in the documentation somewhere, but can someone tell me how to execute an MPX from inside the Prompts editor?
I asked this same question years ago, got an answer, did it, but forgot how!
Thanks.

Normally MPX isn't executed from Prompts menu, they are in Menu editor. The command is GX, but there is now GY for Python as well. You insert (i) a command in the menu editor, it put in the (Q)uit menu entry which you edit
to do what you want. When you (i)nsert a command you get a setup to edit like:

Display Text | 09(B) BBS List & Telnet
LightBar Low | 07BBS List & Telnet
LightBar High | 15BBS List & Telnet
Hot Key | B
Access | Up | 0 Escape | 0 Display Type | Access Down | 0 Tab | 0
X Y | 0 0 Left | 0 PageUp | 0
Timer | 0 Right | 0 PageDn | 0
Timer Type | Interval Redraw | Yes Home | 0 End | 0 Command ------------------ Access ----- Data ----------------------------
(GO) Go to new menu bbslist

If you had a bbslist.mps it would compile with mplc to bbslist.mpx and to
run it you'd change the (GO) Go to new menu line with the tab and up arrow
and edit it to:
(GX) Execute MPL program bbslist
then hit ESC, save the changes? yes
It would then be able to execute an MPL called bbslist.mpx from the current themes script directory by default. You can build up commands in long lists
to do more complex tasks. The commands come from a list that you get access
to then you go to change a command list (GO) above you can scroll through the full list which includes the edit menu command and all others.

--- Mystic BBS v1.12 A43 2019/03/02 (Linux/64)
* Origin: Mystic AlphaTest bcw142.zapto.org:2323 (21:1/145.2)