how can i get maps to show a preferred route

Normally maps will show the 2 or 3 best routes, which will be different based on current traffic conditions. You can tap on any of the highlighted routes to pick the one you want. If you want to take a completely different route just start driving it and Maps will continually show the fastest route from your instantaneous current location. But there is no way to say that you want to drive a specific route. If you know the route why do you need Maps at all.