When I've tried to schedule these routes, I just look at the airport's passenger demand bar. If you hover your mouse over the bar, it'll say for example 56% passenger demand. In my experience, each airport with a % over 50 will not be able to be used as a refueling stop. You'll need to find one with less than 50% to use it.
The demand to and from the refueling stop is irrelevant. You can refuel in middle of Siberia, Russia, with demand from your airport to refueling stop, and from refueling stop to your destination being zero. Once you have the route set up, it will show you the correct demand from your origin to destination. The only thing you care about the refueling stop is that it has long enough runway, it is open at the time you are planning on landing and taking off from there (both ways).
But the way you create the route is you start setting up the route as if you were flying to the refueling stop. On the "middle" screen, where it shows you 2 options:
1- Confirming Routing
2- Optional, select 2nd destination
You go with #2, and find your ultimate destination.