See the question and my original answer on StackOverflow

The FORM element has a special treatment. See here on SO for more: HtmlAgilityPack -- Does <form> close itself for some reason?

So, you could do this:

var doc = new HtmlDocument();
HtmlNode.ElementsFlags.Remove("form"); // remove special handling for FORM
doc.LoadHtml(input);
var body = doc.DocumentNode.SelectSingleNode("//body");

if (doc.DocumentNode.SelectNodes("//form[@action]") == null)
{
    var form = doc.CreateElement("form");
    form.Attributes.Add("action", "/pages/event/10302");
    body.PrependChild(form);
}

but it will get you this:

<html>
 <head>
  <title></title>
 </head>
 <body>
  <form action="/pages/event/10302"></form>
  <p>Full name: <input name="FullName" type="text" value=""></p> 
  <p><input name="btnSubmit" type="submit" value="Submit"></p>
 </body>
</html>

Which is logical, you don't surround anything in that new form. So, instead you can do this:

var doc = new HtmlDocument();
doc.LoadHtml(input);
var body = doc.DocumentNode.SelectSingleNode("//body");

if (doc.DocumentNode.SelectNodes("//form[@action]") == null)
{
    var form = body.CloneNode("form", true);
    form.Attributes.Add("action", "/pages/event/10302");
    body.ChildNodes.Clear();
    body.PrependChild(form);
}

which will get you this:

<html>
 <head>
  <title></title>
 </head>
 <body><form action="/pages/event/10302">
  <p>Full name: <input name="FullName" type="text" value=""></p>
  <p><input name="btnSubmit" type="submit" value="Submit"></p>
 </form></body>
</html>

This is not the only way, but it works, and you don't necessarily have to remove the FORM special treatment.