We had a similar question some time ago in our email support.
I can’t quite remember, but I think there was an app that handled transferring the flight route to Garmin hardware. The transfer to that app was done via the share function directly from the VFRnav route planner.
Could that be? I myself have no experience with Garmin devices.
In VFRnav flight planning, the Share option is in the top-right of the menu. Try going through all the formats there to see whether the Garmin app can be selected in the list below.
I think that’s how it was.
I’ve just noticed that I think I mixed that up. What I had in mind was a Dynon system — not Garmin. That ran via a “companion app” to get routes onto the hardware.
For Garmin we lack documentation for the protocol. I’ll add this to the to‑do list again and check whether we can make progress on it. Unfortunately, we don’t have access to a Garmin device to test the transfer ourselves.