Bind AJax Target control id after the page load using Javascript

Hello ,

I am using autocompleteextender control in asp.net ,but the target control id of that autocompleteextender I want to bind to a runtime control i.e textbox ,and how to refer that completeextender by its id. I want to bind target control id after the complete page load .because runtime controls available after full page load only. How can I do that any one suggest it is helpful.

Thanks @murali