Great, so this can be done. SendToModule needs to be delayed. I couldn’t get it to work in hideBar.sys but in SimpleButton/simpleB.sys that has the app icons.
This is a temporary solution with Schedule. In the forum, somewhere it was mentioned a better way. Add this at the end of simpleB.sys.
To keep things simple, I made it in one file as per manpage example.
# To include the panel in a button
*FvwmButtons: (Panel(down, delay 0, steps 16) \
SubPanel "Module FvwmButtons SubPanel")
# To define the panel as an instance of
# FvwmButtons with a different name:
*SubPanel: (Icon my_lock.xpm, Action Exec xlock)
*SubPanel: (Icon my_move.xpm, Action Move)
Panels are more designed to be shown temporally when opened. If you want a more permanent button, don’t put it in a Panel, instead just have it in its own FvwmButtons or just position it where you want. You could make a button that could hide/show this window. This would probably be a better approach than trying to make Panels work for this.