Remote deicing works in P3D, if you don't see the option it's because:
- You are not outside a parking spot. This should be easy to verify, because the GSX menu will show a different menu if you are in or out a parking: the services menus or the parking selection menu.
- If you see the parking selection menu, but you don't have the option to request Deicing, it's because icing conditions haven't been detected. The latest version that is out since last week has icing always detected provided the temperature is below +4°C
You don't have to set parking brakes either, it's up to you. As long you are outside a parking and icing conditions are detected, you'll always have the option to deice. Of course, trying to do that while in motion might not be a very good idea.