Oh I see, *that* kind of Tag
theducks had the right approach. It's in tweak_book_gui.json.
Close calibre down before manually editing tweak_book_gui.json in a decent text editor (e.g. Notepad++ on Windows, not Notepad or Wordpad). Look for the label "insert_tag_mru" and add your missing tags there. Save file, then restart calibre.