Deploying IIS site in Azure on a different port but using Microsoft provided DNS - how to configure?

Helping a friend with a project where he wants to host a simple site in Azure using IIS (2012R2).

Got the server up and running and IIS installed and running. If we point our browser to the Microsoft provided DNS, it shows the default IIS page

My friend wants to deploy a different site which he's built already on the server but with a different port.

It shows up okay on the server with localhost:777

but we can't work out how to get external access. I have created a network security group to route port 777 to the server (I did it for 80 and 443 also and I see 3380 was on by default of course).

Our external DNS is <name>.australiasoutheast.cloudapp.azure.com

What I have done also is look at IIS binding and try to bind port 777 to the internal IP and use the above as the host name but I get an error invalid host name. If I leave the host name blank it saves but external access doesn't work.

Have I missed out something fundamental?

Thanks