How to create custom lists for Tag Panel pull down fields

I have tried and cannot believe I cannot find instructions on how to create the pull down lists to appear in the tag panel. I have searched the forum and documentation but have not found much other than how to delete a word from the list and previous requests for the feature.

Does one have to input the list one by one? Is there a text file you can edit so you can quickly edit the list? I have a rather long Genre list so if I can avoid going one by one it would be great.

Please note that custom genres could have been defined for a long time, see Tools>Options>Genre

Thanks, I thought the method may have changed with this new feature. I guess it is one by one and no text file. It also seems like you cannot access the list for other fields. I have multiple Genre fields that I like to keep separate in case I want to assign them somewhere else, which I then use to make my main Genre. I would have to input everything multiple times.