How could you add an item to a specific place in a player's inventory, for example the top right corner?
You can get the "UIProfile" as named in the clientData... but I tested that on Classic. Pocket UI is no longer used by the players, not that much, but why not being the same?
Neither the UI scale nor their OS has anything to do with it, sadly. It differ based on your screen resolution.
It would be the first slot, I assume, but the problem is finding the other corner slots. It could be 9 for you, but also 5 for a phone, and 6 for a tablet.