Send variable using html form to php

The problem I have is that when I execute this form from an external page, the input of name token, is not passed. The confirm.php page opens but the echo that I have in it is not executed. I have to say that it is a form that I create in php and send by email. If I generate a form from a page on the same server if the echo text appears. Say that the page from the outside opens and put a text bareback, the text appears. The problem is that the input token is not passed. Could it be a problem with the domain certificate? As if it's worth something, when you execute the submit of the form, you get a message saying that you are going to send information to an external page, you want to continue.

On the cofirmar page I have:

$token= $_POST['token'];
echo $token;

And the page from where I'm calling.

$cuerpo = '<form action="http://privada.mgsehijos.es/preproduccion/php/confirmar.php"   method="post">'.
            '<input type="text"  name="token" value="prueba"><br>'.
            ' <input type="submit" value="Enlace">'.
            '</form>';
             mail($para,$de,$cuerpo,$headers);