How to prevent B team player from playing for 1 game?

I have a cup game and want 3 B team players to play for Main squad. The problem is that the B team has a game on the same day as Main squad. When I get to Main squad selection the B players show as Unavailable because they played a game earlier that day.

How can I prevent the B team players from playing on that day?

Do I have to move them to the Main squad and then move them back to B team after game?

The easiest way is to promote the players you want to use to the main squad until they've played the match you want to keep them fresh for. There may be another way, but promote and then sending them back is only a couple of clicks and would ensure that they aren't chosen for B team selection.

I manage Strasbourg in Ligue 1 and I'm pretty sure I can choose a 2nd team player and set: availability for 2nd team->Next game->Unavailable without having to move them up to the First Team for the furation of the 2nd team match. Also I've noticed that if you set lineups (choose players for positions in Squad View) for your 2nd team or Youth team, your coaches managing those teams usually go with the precise lineup that you chose - even if you're not actually in charge of them. That was often not the case in FM 19 and other earlier iterations.