Javascript - Fill Textbox within ModalPopupExtender

OK, So I have a Checkbox and a Textbox within a ModalPopupExtender Panel.
When the operator clicks the Checkbox...I want to put a value into the Textbox.

For some reason...this code is NOT working:
function GenTrackNumber()
    var today = new Date();
    var mm = today.getMonth()+1;
    var dd = today.getDate();
    var yyyy = today.getFullYear();
    if(document.getElementById('<%=chk_GenerateTrackingNumber.ClientID %>').checked)
        document.getElementById('<%=txt_TrackingNumber.ClientID %>').value==mm+dd+yyyy;
        document.getElementById('<%=txt_TrackingNumber.ClientID %>').value=="";

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== is a comparison check
Just use = to set the value

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