Javascript function is not working on popup button

I have a ModelPopup which contains a textbox. I want to set focus on that textbox when popup appears, So I called a javascript function onclientclick of TargetButton to set the focus on textbox. Now the problem is onclientclick event fires before popup appears and when popup appears focus is not set to the textbox.

So is there any way that I can call javascript function after popup appears. Please help