On the Garmin click on sensors, click power, and select your trainer, making sure it’s switched on and lights flashing to sync it.
It should then be connected to the Garmin unit.
I created a indoor bike setup on the Garmin, select free ride from the menu and it records when you press start button. Press end button when you finish your ride.
After your ride, save both the Zwift ride as normal and then the ride from the Garmin device.
Goon to garmin connect and you will see both rides recorded.
Openthe zwift one with the map and make a note of time, distance and elevation.
Copy the route name if required and the open the indoor trainer ride.
Click the 3 dots top right, select edit.
Change distance and time if required to match with the Zwift ride.
Continue to bottom of page and there is a button to further edit.
Click that to change elevation, calories n liquid if required.
Press save. Add photos if required.
You an then elite the Zwift ride if required as it will double your distance on garmin. 
This workaround seems to work for Zwift events and free-ride sessions.
Link Zwift and Garmin Connect accounts so that the ride uploads to Garmin from Zwift.
Set watch display to always on. Don’t know why this makes a difference but it seems to stop data drop outs.
Go to broadcast heart rate on the watch and turn this on.
Do your Zwift ride.
Let the Zwift ride upload to Garmin.
Turn off “broadcast HR” and “display always on”.
Sync the watch to Garmin Connect twice. (Some calculations are performed in the watch so need to be re-synced to Garmin Connect to dispaly correctly.
Bingo. all data present and correct.
Caveats.
Don’t start an activity in Garmin, just let it broadcast the HR. It doesnt need to be received by anything, just broadcast. (If you get interference on BT or LTE you can stop broadcasting, but make sure that the display is set to “always on” or you will get data drop outs if the watch display goes to sleep for more than a few seconds.)
Trainer is a Jetblack volt, running through a WIndows PC via Bluetooth, Garmin is a Fenix 8.
I dont know about any other hardware combinations.
I use a Jetblack HR chest strap to push the HR to Zwift via the trainer so I can see on screen rather than looking at the watch, but this data doesnt carry through to Garmin on the upload. If you use the broadcast function from the watch as well then Garmin seems to merge this with the Zwift data and updates all the recovery and body battery stuff correctly.
I am still playing around with this but this seems like a good solution for races and rides if you are using Zwift to run your trianing program rather than Garmin. If you are trying to capture all the workout data in Garmin, ie percieved effort etc. not just an activity this seems to be more problematic.
Hope this helps someone. RideOn!