I have to apologize, my memory fades: ...
in fact, the dialog "Mp3tag Options/Tag Panel" does not allow to define a dialog for the GENRE tag field for the second time.
You have to manually change something in this file: "%Appdata%\Mp3tag\data\usrfields.ini"
For example, put in a new section:
[#5]
value=GENRE
name=Genre (edit field)
multiline=
Doing so the Tag Panel will get the additional dialog field.
See also ...
multiline comment field in the tag panel
DD.20180130.1311.CET