How to place a form like this on a page:

<form id="my-custom-form" action="https://www.xyz.com/form.aspx"" method="POST" runat="server">
  <input type="hidden" name="parm1" value="val1" /> 
  <input id="Email" maxlength="40" name="Email" size="20" type="text" /><br />
  <input type="submit" name="submit" />
</form>

By default, Sitefinity places a form on a page to render all widgets in. Because of this, additional form tags are stripped automatically when inserted manually via the HTML markup of a Content Block for example.

1. Extract the attached script.
2. Place a javascript widget on the page where the form is placed and use the javascript file.
3. Modify the following lines from the javascript file using the information from your form:

var container = document.getElementById("form-id") // Place the id of the form (e.g. my-custom-form) 

form.setAttribute("action", "form-action"); // Place the action of the form (e.g. https://www.xyz.com/form.aspx)
form.setAttribute("method", "form-method"); // Place the method of the form (e.g. POST)

4. Modify the form by replacing the form tag to div.

A workaround would be to create a master page with this form already incorporated and use that master to create the Sitefinity webpage.

For example:

<%@ Master Language="C#" AutoEventWireup="true" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">;
<html xmlns="http://www.w3.org/1999/xhtml">;
   <head runat="server">
      <title></title>
   </head>
   <body>
      <form id="my-custom-form" action="https://www.xyz.com/form.aspx"" method="POST" runat="server">
          
         <input type="hidden" name="parm1" value="val1" /> 
         <input type="hidden" name="parm2" value="val2" /> 
         <asp:ScriptManager ID="ScriptManager1" runat="server" />
         <div id="Header">
            <asp:contentplaceholder id="Header" runat="server" />
         </div>
         <div id="Content">
            <asp:contentplaceholder id="Content" runat="server" />
            <input id="Email" maxlength="40" name="Email" size="20" type="text" /><br />
            <input type="submit" name="submit" />
         </div>
         <div id="Footer">
            <asp:contentplaceholder id="Footer" runat="server" />
         </div>
      </form>
   </body>
</html>

Please note that this workaround will prevent you from using any other Sitefinity forms on the page.

Another workaround would be to create a custom control and handle the post request in the code-behind.

Sitefinity Documentation: Create a template using a Master page - http://docs.sitefinity.com/create-a-template-using-a-master-page