My editor does have an ID of Editor1 so Editor1.ClientID returns

'CE_ctl00_ContentPlaceHolder1_Editor1_Editor1_ID'

 

So I've used the script as-is from your example. 

 

by using editor1.getElementsByTagName("IFrame").id i managed to find the editor's IFrame id.

However using:

 

'document.getElementById('<%=Editor1.ClientID%>_Frame').onclick = helloworld;'

 

still doesn't fire the alert when clicking inside the document.