In case you are planning to use Azure Application Gateway to receive traffic from different clients address like client1.testdomain.com, client2.testdomain.com, client3.testdomain.com etc, then unfortunately you have to create separate listener for each of the client. The Multisite listener doesn’t support wildcard in its hostname field as shown in screen below-
This limitation becomes huge because there is limit of 20 listener per Application Gateway and you can have upto 50 Application Gatgeways per subscription, which will allow up to 1000 listeners per subscription.
Lots of people have requested for this feature but no update from Microsoft product management team about it- https://feedback.azure.com/forums/217313-networking/suggestions/19527121-application-gateway-support-wildcard-hosts-in-lis
I hope this feature is enhanced to match up with what other competitors in market are providing.