Use old account on a Steam Deck

I have an old account that I used for war thunder before downloading it on Steam, but when I downloaded it for steam deck and started it, it opens an account created with my steam account, but I want to play with my original account, not the steam one.
On PC it is very simple because you change the user and you see the screen to put email and password, but in steam deck is not so :( I open a window and change account but when I return to the game I’m still in the same account, does anyone know how to make that change?