Please, please add Capo Chord Symbol Definition to the list of eligible key commands.
You might be able to create your own key command for this by editing the key commands JSON file.
Just search the forum for “JSON” and I’m sure you’ll find the procedure somewhere. Just make sure you save a copy somewhere first as a backup!
It’s actually not possible in general to open this dialog via a key command, because it relies on knowing which player is selected in the Players panel in Setup mode. You can’t know the internal ID of each selected player, so you can’t create a custom shortcut in the JSON file.
However, we can change this in future to make Dorico able to figure out the selected player on its own, in which case you would be able to assign a key command to it.