I know how to create one. It is MANDATORY important to create a "new" declarative action from the right place.
This article helped. Specially this paragraph
What I miss and found nowhere though really:
What is the structure of the event mapping?
How can I call "script" with an event?
How can I open a new browser window? (maybe with script)
After I finally, after days of searching, found the right place to "create" a declarative action for a related list, it was 5 minutes of work to get the export for records in the related list working.
Create a declarative action in the related lists section. Add EXPORT_LIST as Specify client action. Define table of the related list. Done
