How do I choose a dialog option with keyboard, but without TAB? I want to choose "Add".

I guess I can enable "Keyboard access" and use the tab key, but there must be a reason developers have it disabled by default. And that answer says there are dialog-specific shortcuts anyway, and I guess that's the "Mac" way to handle it. What are they in this case?

(Tried various combinations of ⌘, ⌥, ctrl, ⇧ and other keys. Only ↵ and ⌘+. choose "Don't Add" and that's it.)
Try  Cmd ⌘   A  or  Cmd ⌘    Shift ⇧   A  

They're a bit hit &amp; miss as to which works in any given circumstance, but [key/keys] + [first letter] will usually do it.

btw, you can toggle the Tab behaviour on &amp; off at will with  Ctrl ⌃    F7  even whilst the dialog is open.