how to do validation of dynamically added textbox?

 hi to all,
i have added textbox dynamically which is located at modal popup. i want to do the validation for that textbox.
i have written javascript for validation and assigned to that control in codebehind as follows,
textbox1.Attributes.Add("onchange","chkValidation();")
but the javascript is not tget execute.
am i do right thing? or how to i do validation of that textbox?
 
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