You write a custom field with the name LABEL and the content "White Label".
Both are written into your mp3 file as you can see in Alt + T in Mp3tag.
But I don't think that Serato or Rekordbox read this custom (= non standard) field and its content.
You can try it the other way:
Write your content "White Label" in Serato or Rekordbox and save it.
Then you can open the same mp3 file in Mp3tag and press Alt + T again on this file.
This way you can see in what field name your 3rd party software writes your information.
Use the same field in Mp3tag to fill your label information for example.
In an old thread from 2008 someone says that the field name you are looking for could be PUBLISHER. You would have to try, if this still works for your current Serato version.