App.clientwebsite.com maps to app.mywebsite.com - got a bit stuck!

Hey clever people. I have built an app that sits on AWS via Forge. There is a subdomain called surveys.myapp.com.

I want to map clients’ domains to my subdomain so that (for example), surveys.FredsBakery.com serves up surveys.myapp.com.

I can get the client domain to resolve (via a CNAME on the client DNS) but forge loads a 404.

I feel it’s something to do with the ‘servers’ key in the Nginx but I don’t know how to fix this.

Any ideas?

Ps. Yes, the wildcard subdomain is checked.