I need to forward requests from an external domain/server, i don’t recieve their IP adress (probably dynamic). Which WAF rule could i use to forward everything which comes from “license.helloworld.com”
You can’t do that as that information is not sent in the request. You may find it in the referer but that’s not guaranteed. You’ll probably need to ask them for their IP addresses or use some other method (header, user agent, UUID, specific URL, etc) to use in a WAF custom rule.