How to set focus in TextBox after JavaScript alert box is closed

Check this sample

<asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel runat="server">
        <ContentTemplate>
            <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
            <asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="btnSubmit_Click" /></ContentTemplate>
    </asp:UpdatePanel>

    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", string.Format("alert('Employee id Exist');document.getElementById('{0}').focus();", TextBox1.ClientID), true);
    }