How to assign in the new TR platform an outdoor ride, i.e Strava ride or “.tcx” & “*.gpx” files?
You can add the outside ride as outlined here: https://support.trainerroad.com/hc/en-us/articles/360015964912-Add-Individual-Workouts-and-Create-a-Custom-Plan
But you can’t assign a route to an outside ride ahead of time.
Once you do that ride the data will be synced from Garmin/Strava automatically if you setup ride sync here: https://www.trainerroad.com/profile/sync.
Then the ride will automatically match with the workout.