I created a custom 3D platform (Helipad) a notch above ground level and I selected the Collision Material (not road material).
In Dev mode, I added a Helipad and made it at the same height above the ground as the 3D platform.
I see the Helipad in MSFS as an (H) icon on the Map, I select it as a start position, when the flight loads and I see the screen with the Yellow button (read to fly), I see the aircraft sitting on the 3D platform.
When I click "ready to fly" the aircraft is teleported to the closest airport.
Just to confirm: I guess adding a Helipad is not an official start position in MSFS although we see the (H) icon on the map therefore we always need a runway as a start position (make it transparent in this case)?