I'm afraid not... https://www.reddit.com/r/FifaCareers/comments/1mjexha/fc_26_formations_are_the_exact_same_with_no/
Many people thought this was a problem with FC 25, but EA didn't do anything about it. Just minor adjustments in career mode stating they "listened to the player's feedback". Yeah right...
It's pathetic that something as basic as creating your own formation is impossible, especially because they had it before! It makes a lot of team's composition of players unusable in one formation, so you just have to sell a bunch of midfielders or wide players to make it work, and it limits your choice in buying players throughout the whole game.
Trying to defend your one goal advantage by subbing your CAM with a CDM and move it further back? No chance. It gets even worse when you go down to 10 men, good luck finding a suitable formation then. But I guess no real life manager ever does that anyway, right?