Custom Form and 404 Error

Hi

I’m brand new to WordPress so excuse the basic questions ?? I am trying to add a contact form to a page. I want to do this manually rather than using a plugin mainly as I’m just trying to get my head around the whole process. Initially I want to avoid custom templates and just add a form into a standard page. I am pretty much working from the post at https://www.badlydrawntoy.com/2009/10/07/how-to-run-custom-php-code-in-wordpress-the-right-way/.

So far I have added a plugin that allows me to run PHP code on a page. The code I have added simply contains the PHP command…

require_once(ABSPATH. '/wp-content/custom-php/p2.php');

p2.php conatins the following…

<form method="POST" action="<?php echo $_SERVER['REQUEST_URI']; ?>">
  <div>
    <label for="name">name</label>
    <input type="text" name="name" id="name" />
  </div>
  <input type="submit" value="go" />
</form>

If I go to my site and access this page it opens up and shows the form so the include is working … so far so good. If I click on the submit button with nothing in the input field then everything is still happy. The minute I put some text into the input field and click submit I get a 404 Page Not Found error. Can someone please suggest what might be going on?

Thanks

Alan