Read Thread: Boxes on a route?
Check out Trip planner after going to Toolbox on the toolbar at the top of this page.
If it is a specific highway, you can go to simple search and use the trip search.
The easiest way for me has always been to do searches of the towns I will be passing through or near. You can expand the miles out from the town. Or not. This had always worked well for me. I’ve never cared for the trip planner. But that’s just me.
Go to the Trip Planner under the Toolbox pulldown menu. There are dozens of pre-defined routes available, mostly along major roads. If you want to create your own you can. I just recently created a trip from Arlington, Tx to Newport, RI for a road trip this summer. After inputting the start and end points and a trip name you can add features and adjust the route on a map that comes up of your route. After it looks the way you want save the route. AQ chews on it for a while and then you have a route! After that you can search for letterboxes along the route (within 5 miles of the route is the default). I take the search results and download them to my GPS as a .gpx file but that is a whole 'nother tutorial.