You need to update the nginx file on the server to do this. You have an option in forge to do that per site, or you can SSH into the server and do it yourself on the command line.
Hi, thank you for the reply. I tried that but unfortunately due to restrictions to the way the servers and old app was set up, I have some code that distinguishes the environment then sets the subdomains.
Your solution is not very robust. It takes https://my-app.uk/some-url and changes it to the www version, but strips off the "some-url". You always get redirected to the homepage, so you don't end up going to where you actually intended. That's bad UX.