For refuelling leagues the trigger to make the car pit is
not number of laps, auto pit stop is triggered by fuel remaining. When your fuel level becomes low enough that your car can't complete another full lap it will pit, fit the compound of tyre that you selected in the main strategy and load fuel to the number of litres you have specified.
This is what the strategy page looks like. It is similar to the UI you are already using, the difference being there is no field on this screen to select fuel load.
Instead you select both the tyre compound and fuel load for each stint. You need to decide how long the stint is to be (in laps) then calculate how much fuel you would need to complete this number of laps. The game will give you an estimate of the number of laps, but bear in mind that this is at PL3 and you must have set a practice lap so that the game knows what your fuel consumption will be. If you try to set strategy before you ran a practice lap the fuel/lap estimation will be very conservative.
Yes, you can adjust your strategy "on the fly" during the race, but of course, unless it is raining, your first stint tyres can't be changed and the stint can't be made longer than your original plan because you already set the fuel quantity before the race begins.
