The Prosim 737 is not internally supported by GSX, so it relies on making a custom airplane profile. If the airplane profile has specified the airplane doesn't use a default Fuel system ("Show Fuel/Cargo page" DISABLED), the fuel menu won't appear.
Now, it's not as if you can just change it as you wish, because it depends if the airplane CAN be refueled externally and, since the Prosim has a custom systems simulation, I guess it likely can't so, you should keep the option Disabled, which is why whoever made the airplane profile you are using set it (correctly) that way.
