I. Make sure your game is set to use the Habbo Hotel Origins XL client. This can be chosen the first time you boot up the game or by clicking the cog icon on the steam library page>properties>select XL client from the drop down menu

II. Find the XL client .exe, usually in D:\SteamLibrary\steamapps\common\Habbo Hotel Origins and look for HabboHotel-ous-xl.exe. This is the English .exe for the XL client.

III. Select properties>compatibility>change high DPI settings>tick the box for 'Override high DPI scaling behaviour>make sure the drop down says 'Application'>click Apply

DONE! You can now open this game and enjoy it in fullscreen!
You might have to change the settings for your taskbar so that it hides automatically but other than that everything is good to go!