Actually it seems you don't.
Thats a UI bug in Win11. You are using 120% as entered into the box on your second screenshot. For some reason Win11 gets stuck showing the last scaling value you picked from the standard list rather than showing you the actual one you entered on that disabled drop down.
Rather confusing really.
The good news is the issue has been reproduced and is being caused by this scaling factor being an odd value. 125% will probably work just fine just not 120%. We have this fixed for the next update.