Hi Liam,
I am Dave, I will help you with this.
The WIndowApps folder is the most protected folder in Windows to ensure the integrity of the Windows UWP apps and Store apps and games, changing permissions on the WindowsApps folder will most likely corrupt all Windows UWP apps and Store games, many people have found that all the apps and store games stop functioning correctly and they had to re-install Windows.
What do you need to be able to add the game to Steam, is it the path to the game exe file?
If it is, open Task Manage, then select the Processes tab.
Right click any column header and turn on the Command Line column
Then start the game and in Task Manager on the Command Line column you will see the exact path to the game executable.