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There seems to be no way to add an action between two horizontal separators in a custom panel.
you need to delete one of the separators then create an action and then re-create the separator... cumbersome.

Peter,
If you have two horizontal separators together, make sure they are both displaying the minus symbol, then Right Click between the separators just inside the panel's right border/margin and you should see the popup that offers Paste, Add Action and all the other options. Its a bit fiddly, but works.

Thank you!
Fiddly indeed, the Custompanels could do with some improvement...

one thing i'd like is a slider to scroll down when a custompanel gets too long.
now the bottom goes out of frame and the only way to scroll down is to place it in the side drawer...