Set focus on editor

I want to set focus on editor when one button on my page is pressed. I'm using fckeditor for .NET, but need to do this with javascript. I tried to write document.getElementById("fckeditor").focus(); But the ID of the fckeditor is changed to "ctl00_ContentPlaceHolder1_PageEditor_fckeditor___Frame"when i look in the source code of the HTML Page, because im using contentplaceholder where the editor is places inside. How can i solve my problem? PLease help!