You will need to change the binding for the virtual website. You should have the MyApp listed in the tree down the left hand side of the IIS console. Select it and then use the Bindings link of the far right side of the console.
Add your domain name and link it to the IP address of the server.
If you MyApp is already a folder underneath a virtual website of myserver.mycompany.com then you would be better off moving it out to its own virtual website
You can see more about host headers and binding them here - http://technet.microsoft.com/en-us/library/cc753195(v=ws.10).aspx