YES!

Thank you! I had to modify it slightly do to my application being written in VB.NET, but I have gotten it working now!

Here is the code that finally worked:

[codebehind]

 

If Not Page.IsPostBack Then
            If (Not Me.ClientScript.IsStartupScriptRegistered("Startup")) Then
                Dim sb As StringBuilder = New StringBuilder()
                sb.Append("<script type=""text/javascript\"">")
                sb.Append("Sys.Application.add_load(modalSetup);")
                sb.Append("function modalSetup() {")
                sb.Append(String.Format("var modalPopup = $find('{0}');", ModalPopupExtender1.BehaviorID))
                sb.Append("modalPopup.add_shown(SetFocusOnControl); }")
                sb.Append("function SetFocusOnControl() {")
                sb.Append(String.Format("var textBox1 = $get('{0}');", txtUsername.ClientID))
                sb.Append("textBox1.focus();}")
                sb.Append("</script>")
                Page.ClientScript.RegisterStartupScript(Page.GetType(), "Startup", sb.ToString())
            End If
        End If

  Thank you all for your assistance with this issue. Hopefully this thread will be useful to others in the future!

Sincerely,
KJAK