Maybe this will help,
https://docs.mendix.com/ats/howtos/ht-version-2/custom-action-basics-2
some features are a little bit hidden and the search fucntion in the top searches mendix globally so you wont get many results for ATS there, but on the left you can skip through several use cases on how to make custom actions with (unsupported) widgets in ATS.
Hey,
There is also some documentation for how to create a custom action for a dropdown: https://docs.mendix.com/ats/howtos/ht-version-1/cab-05-reference-selector
Hope this helps!
Kind regards,
Myrthe