Please try the following code:

 

<Script Language="javascript">
    function callInsertImage() 
    { 
   var editor1 = document.getElementById('<%=Editor1.ClientID%>');
            editor1.FocusDocument(); 
            var editdoc = editor1.GetDocument(); 
            editor1.ExecCommand('new');
            editor1.ExecCommand('insertdocument');
            InputURL();
            document.getElementById("docFld").focus();
    }   
   
    function InputURL()
    {
  var editor1 = document.getElementById('<%=Editor1.ClientID%>');
        var editdoc = editor1.GetDocument(); 
        var links = editdoc.getElementsByTagName("a");      
        if(links.length>0&&links[links.length-1].href!="") 
  { document.getElementById("docFld").value = links[links.length-1].href;      
  } 
  else
  {
   setTimeout(InputURL,500);
  }  
    }      
</script>