That’s one frustration with Scenic. You do it from their web importer.
https://scenicapp.space/webapp.php
From there you can import Supported Filetypes:
GPX: GPS exchange format
KML: Keyhole Markup Language (Google Earth, Google My Maps, et al.)
KMZ: Keyhole Markup Language Zipped (Google Earth, Google My Maps, et al.)
GDB: Garmin DataBase (Garmin Devices, Garmin Basecamp & Mapsource)
ITN: TomTom Itinerary (TomTom Rider)
Or you can paste in the URL from a Google maps browser session where you’ve created a route. If you create a route in Google Maps (web browser, not app), you’ll know you can add locations and also drag the blue line for the route with the little white dots. Once the route is as you like, copy the entire address bar from the browser. Paste it into the web importer. The locations will become Stops (must visit and will always be directed to them and won’t be auto-skipped) and the white dots are Vias (will try and guide you there but can auto skip if you have the option enabled)