OK, that makes sense. But I really think this needs explicitly documenting.
Here is what gave (gives) rise to my misunderstanding.
You say "it is populated with nothing". But that's not the case. Every time you open that panel it is full of keywords. And that list of keywords is, to the extent that one notices from a cursory glance, the same from session to session. That list has been apparently stable with me for months, maybe years. A reasonable scenario would be (assuming that it is not populated with the master list) the list that was in the panel at the end of the last session. But a quick experiment with snapshots shows that that is not the case. There is some default list of keywords (keywords that I am the origin of) that gets filled into that panel every time PM is started. I would still like to know what that list is.
Underneath the panel is a text input field, and below that are three buttons, the left-most one being "Add". If ones enters a keyword into the text input field and then presses "Add", then the entered keyword is added to the list in the panel. That is the "Add" button uses the text input field as its input. On that basis, I made the quite reasonable assumption that the "Add to Master" button also looks to the text input field for its input. You are now telling me that "no, that button looks to the selected keyword in the panel as its input". OK, that's also a reasonable choice. But I don't think that it is the unambiguous obvious choice, a priori; it certainly wasn't for me.
I guess all I am saying is that given the ambiguity in the UI, you really need to provide some documentation. I would really prefer to be able to look something up in the manual than to have to resort to a support forum. One expects in a manual, documentation of all of the various menu items and their associated panels. And if my memory serves me correctly, you used to have a manual like that.
