Make sure you’re using the ConnectIQ app, they kinda (quietly) removed the ability to do this from the Garmin Connect mobile app. In the ConnectIQ app, select your Edge device and go to My data fields > MPA/Power. Select ‘Settings’ and enter your Xert user/pass. Save.
Next time you start an activity, the data field should fetch your signature from Xert and update the dot to green!