[Koha] Country subfield in framework

[Actiu informatica]

I'm editing subtags in a MARC21 framework, and trying to use 044.c for
book's publication country.
At Tag subfield structure -> Edit -> Other options; I set authorized
value "COUNTRY" and thesaurus "GEOGR_NAME"

Then, when using this framework on cataloguing a book, this subfield
appears with a dropdown box with "No matches found".
How can I prepare values to this box be linked to?

Thanks.