I don't have a good answer or a source for a tutorial, but I can at least share a couple discoveries I made while fiddling with this recently. For reference I am doing this on MacOS Sequoia 15.5.
It appears that whatever file name is put into the user actions box is run from its directory (not the directory of the `.ioc` project file) using some kind of restricted native shell. Path and environment variables appear to be missing, but it's unclear to me exactly what the nature of the shell is.
I first attempted to use a python script to make the post-generation steps platform agnostic, but it just failed silently, giving me no output in the Output pane at the bottom of CubeMX. The script was executable and should have been able to run as it had the if __name__ == "__main__" hook, so it might be that there was no python on the path for the terminal executing the script.
I switched over to a .sh script and got success. The stdout (unsure about stderr) terminal output appeared in the Output pane of CubeMX, as did what appeared to be the exit value of the script, (e.g. ErrorLevel:0)
Hopefully this helps for anyone still looking to use this feature, but an official tutorial would certainly be better than my speculation :)
