This is my preferred method of transferring my planned motorcycle routes, which I have prepared using the Scenic Navigation app on my iPhone .
Unless you use Garmin Base Camp planning software everyday and are familiar with it , I wouldn't bother with that software !
Get some planning software on your mobile device, in my case I subscribe to the Scenic Navigation App.Very easy to use and you can plan a days ride over a cup of tea at breakfast, its that simple to use.
But whatever your preferred navigation app is , I will show you the best way, that I know, to get your designed route onto the Garmin Zumo XT, without the Garmin software possibly doing something to I and corrupting the route.
So we will convert the track after import to the Zumo XT, to a trip. Literally takes 1 minute once you know how to do it.
https://scenic.app
https://www.garmin.com/en-GB