User Defined Tags like Genre

In Mp3Tag I can add user defined genres, which is great but I would like the same ability for other tag fields. I have a Format field for instance and would like to add user defined formats like "Album, EP, Single, Compilation, Live", etc.

Is something like this possible?

see here:

That doesn't seem to be what I am looking for unless I am misunderstanding.

There is no function to get user-defined dropdown lists.
This feature has been requested umpteen times.
I used the search function and found the linked thread that shows you the alternative - and also this one from even longer ago with the same advice:

Create actions of the type "Format value" for the field - and - as a help to group these actions together - use the # character in the action name as separator for a submenu.