deicing with engines running on a remote stand is not working. GSX says the engines must be shut down before ground services can be used... In MSFS 2020
If that stand has been defined as a parking spot in the scenery, GSX will assume it's a parking spot, so it will ask to turn off engines.
Remote deicing can be done "anywhere" without adding a custom Deice area, but it must be outside of an existing parking spot, and even if you define a custom Deice area there, it's still a parking spot, so GSX will give precedence to that, in order not to "steal" a parking from the airport.
A way to do that, would be editing the Distance Threshold value (lower the value) of the parking spot, so GSX won't consider the airplane parked unless you are exactly on that spot, then setup a Deicing area in a way that it won't overlap the distance threshold of the parking spot (the green circle), so GSX will allow you to use both.