If you assign Motion to be the “editor” application in the FCP System settings it will open in motion that way. But you really should use the send to command I think, leaves the editor app to be some other app possibly.
In order to get audio to go as well as the video be sure to select it in the timeline window… it does work just fine here… and using this method you can embed the Motion content… allowing you to just double click it later for changes… sort of like a nest but it opens the Motion project file when you do things this way allowing for a true round trip.
Jerry