How to create a custom background in TextMeshPro?

Hey, I want to create a custom background for part of my text.
I want to display key a player needs to press with other TextMeshPro text like so:
but I want to have a custom image background

in the marked area.
I know I can insert an image glyph, but I would have to prerender all possible keys, which is not ideal.
How can I achieve it?

 

I'd probably do it with separate image components underlaid behind the text. You can use the various CharacterInfo APIs from TMPro to get the exact position/sizing etc. for the characters.

Or insert the empty image glyph in the text and use separate text components to display the letters on top.