Looks like the old thread on this died, so reviving it.
None of the Garmin GPSs appear to have a USR page, or any way to input or save a user-defined waypoint.
I fly real world for a certain volunteer SAR and military support organization, and was hoping to practice my routes in MSFS, as well as check out the airspace and potential hazards along those routes, before heading out to fly a support exercise next week. No dice…the routes are a bunch of custom waypoints, and there’s no obvious way to put them in.