How to get a read only text box value from a form and pass it to email body in c#

I have a textbox form that displays a date value from a SP. And I need capture the date that showed on the textbox and write it to an email body.
<asp:TextBox ID="txtInserTS" runat="server" Width="381px" ReadOnly="true" Type="Int32" CssClass="readOnly"></asp:TextBox> I need to add the value of the "txtInserTS" into an email body so that it can be sent to an email address upon a sumbit button click. Here is what I am working on under the code behind for the submit action.
protected void btnSubmit_Click(object sender, EventArgs e) { string txtInserTS = (string)Session["InserTS"]; } Here is the email body method:
private static string GetEmailBody() { StringBuilder builder = new StringBuilder(); buil ...