doamin replacement irule

This is what i am trying to acheive. not getting my logic correct in irule.

User types in f5.api.domain.com , f5 need to replace that with f5.mydomain.com and send it to the back end server ,

and when backend server sends the response F5 need to again replace the request with f5.api.domain.com as client should not see the anywhere the internal domain in the header.

when HTTP_REQUEST {

       set external_hostname "*.api.domain.com"        

       set internal_hostname "*.mydomain.com"   

       HTTP::header replace "Host" $internal_hostname       

    }   

      when HTTP_RESPONSE {        

       if { [HTTP::header value "Location"] ends_with "$internal_hostname" }{

           set location [HTTP::header location]

           HTTP::header replace location $external_hostname

       }

    }