In settings we have an option to select a location, this now needs to be able to have a secondary dropdown location to set a default sublocation. So the secondary dropdown needs to show the sublocations of the option selected in the first dropdown.

Looking around I see only answers that tell me to write a function in js, but the issue for that is that the js file is automatically generated with T4 and the .ttinclude files belonging to that.

I don't really understand this, but it looks like it's not set up to add functions yet, how can I do this?