If store exists then add the store_id

Hi, when an admin creates a store the store_id field can be null but if the admin gives a store_id I want to add it to my coupon model. like $coupon->store_id = $request->store_id. But I want to check if the store exists with that id. I can do it by saying store where id == store_id but is there any way to do it with $request->validate?