I've been using Mapquest with a Garmin 765t for years. Just plot out the route as normal, connect your Garmin to the PC, and select send to GPS as a route. Yes a plug in is needed and you'll be prompted if its not installed. On the Garmin, go to you Data, and select the route and import it. The route you created will now be in you custom routes. If you use enough waypoints, it will be exactly what was created on Mapquest.
Now to be honest, Google maps is much more user friendly than Mapquest. So I normally quicly plot out the route on Google then duplicate on Mapquest for the import.
BTW - been using the Nuvi 765t for the last 4 years on 3 different bikes without issue. It even becomes fully waterproof by slipping a ziplock sandwitch bag over it.
. However, it is having some Bluetooth issues lately so as good as it has been, looking for a replacement.