[MP4] Where to store IMDb, TheTVDB and other IDs?

For movies and TV shows, Kodi and some other software support IMDb and similar IDs stored either directly or as URLs pointing to the respective sites inside simple textual or XNL .nfo files that either have the same file name as the media file with a different extension or have a special name like tvshow.nfo.

I would like to store such IDs inside the media files themselves, i. e. in their MP4 tag. Which field(s) should I use?

By the way, Kodi v18 Leia (source code) does not seem to expect it in any particular field yet.

see here the list of supported tag fields:
http://help.mp3tag.de/main_tags.html
Take your pick.

I know that page of course, but Iʼm looking for advice from people with the same experience.

A good way is to get a file that displays nicely in the player and check for that file the extended tags dialogue in MP3tag.
This should reveal where the information can be found in the tags.