Rule 355.17
If a Game Effect has introduced a mandatory requirement on any of these choices, that mandatory requirement must be followed if possible. A mandatory requirement is one that specifies what choice can be made or how that choice is made, but doesn't specifically disallow other game states. Example: Here to Help is a spell that reads in part “You may play a unit from your hand to a battlefield you control.” If its controller decides to play a unit, they must play it to a battlefield they control. Example: Rek’Sai, Swarm Queen is a unit that reads “When I attack, you may reveal the top 2 cards of your Main Deck. You may banish one, then play it. If it is a unit, you may play it here. Recycle the rest.” The “you may play it here,” portion of this ability is not a mandatory requirement, because the player isn’t required to play the unit to that location.