Unsure on your question specifically. it didn't work for me on FM22, i went years without my pretty decent B team ever entering one of the playable lower divs. This was in a country where B teams were in lower divs along side first teams, so not a dedicated B league, whether that makes a difference.
this feature highlighted in one of the official blogs, but again, whether u18/u23 also stretches to B teams im unsure.
If you've only just started your save, i'd suggest restarting and downloading a league extension so it includes the division your B team is entered in.