MP4 tags - TVSHOW tag not available?

Hey everyone. I looked through the FAQ and searched the forums but was unable to find an answer for this. Hopefully someone can help out.

When I go to edit the extended tags of an MP4 file I am able to select TVEPISODE, TVEPISODEID, TVSEASON and TVNETWORK. However, I am unable to see a choice for TVSHOW. According to the tag mappings list (http://help.mp3tag.de/main_tags.html) I should be able to edit TVSHOW and TVSHOWSORT.

Why are these tags not available to me?

Although in "Mp3tag\help\main_tags.html" tag-field names for MP4 have been declared as available and supported, some tag-field names are missing in the picklist of the editfield "Field" in the dialog "Extended Tags...".
Once you have typed into the editfield the string "TVSHOW" and a value has been entered too, ...
and the edit dialog has been closed by "OK", then the picklist got the new entry "TVSHOW".

You should check, whether this tag-field TVSHOW (tvsh) can be read by other applications too.

DD.20140402.1306.CEST

That worked. Thanks! I thought I had already tried that but I made the incorrect assumption that if the tag is not in the list it would not work.

Does this mean that custom tags are possible for applications that support them?

Yes, you can enter any custom tag into V2.3 and higher tags. The problem is, as you say, is that you have to look for applications that support them.