locate cursor in specific textbox

hey, i build a web chat.
i want that in the chat window that locate in update panel, when the window is load the cursor will be located in specific TextBox,
i tried to use both options: default focus in the client side and set focus in the server side (c#), work fine for only one window.
the problem is when i opend a multiple windows (even more then 1 window) the cursor is start jumping from one window to another
without touching anything.
i hope that someone could help me
Thanks
 
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