重写ASP NET Web窗体操作

我有以下网页表单:

工作形式:

<form id="transactionParameters" method="post" runat="server" action="remote_url.aspx">
        <fieldset>
            <p>
                <label for="Username">Username:</label>
                <asp:TextBox id="Username" Text="TomSelleck" runat="server" />
            </p>

            <asp:Button ID="SubmitForm" runat="server" onclick="SubmitForm_Click" Text="Button" />  
        </fieldset>
</form>

这就成功地提交了表单值 Username 到 remote_url.aspx 并将用户带到远程页面。

我面临的问题是,在提交表单和重定向用户之前,我需要为表单添加一个值,例如:

所需功能:

<form id="transactionParameters" method="post" runat="server">
        <fieldset>
            <p>
                <label for="Username">Username:</label>
                <asp:TextBox id="Username" Text="TomSelleck" runat="server" />
            </p>

            <asp:Button ID="SubmitForm" runat="server" onclick="SubmitForm_Click" Text="Button" />  
        </fieldset>
</form>


// This method is fired but I don't know how to execute the commented out code
protected void SubmitForm_Click(object sender, EventArgs e)
{
    // Form["Username"] = Username.Text.Uppercase();
    // Form["NewValue"] = DateTime.Now.ToString();

    // ExecuteAction(Form, "remote_url.aspx");
}