URL Rewrite/ProxyPass Functionality

you would need to create a pool with a FQDN node of using Azure site "app1.azurewebsites.com". Assign that pool to VIP of abc.com. Add below iRule to the abc.com VIP for Host header rewrite. This should do the trick.

when HTTP_REQUEST {
      HTTP::header replace "Host" "app1.azurewebsites.com"
      }
when HTTP_RESPONSE {
    HTTP::header replace Location [string map -nocase {app1.azurewebsites.com abc.com} 
    [HTTP::header Location]]
}