Unable to select user property inside remote config in firebase, it is greyed out

You must first add the user property to Firebase -> Custom definitions.

1. Go to Firebase -> Custom definitions
2. Click Create custom dimensions
3. Select Scope to be User
4. Complete the remaining fields (note you can't add user_id)
5. This step might be optional but had to wait a few hours for it to appear.

Currently Firebase doesn't seem to allow for the built-in/automatic user properties to be allowed here. user_id would be a really good example of something you might want but can't have.