Can this be used in a popup?

Yeah, here is the php code that I used.

This is the code that we add in the popup (as html)

<form action="link-to-php-file" method="POST" accept-charset="utf-8">
<table>
  <tr>
    <td>
	<label for="name">Name</label>
	<input type="text" name="name"/>
    </td>
    <td>
	<label for="email">Email</label>
	<input type="text" name="email"/>
    </td>
    <td>
	<input type="submit" name="Subscribe" value="Subscribe"/>
    </td>
  <tr>
</table></form>

And here is the php file that you got to create and add it on the server.

<?php
	//--- You need to set these ---//
	$your_installation_url = 'http://www.your-website.com'; //Your Sendy installation (without the trailing slash)
	$list = 'list-id'; //Can be retrieved from "View all lists" page
	$success_url = 'http://www.your-website.com.com'; //URL user will be redirected to if successfully subscribed
	$fail_url = 'http://www.your-website.com'; //URL user will be redirected to if subscribing fails
	//---------------------------------------------------------------------------//

	//POST variables
	$name = $_POST['name'];
	$email = $_POST['email'];
	$boolean = 'true';
	//session_start();
	$_SESSION['ref'] = $_SERVER['REQUEST_URI'];
	echo($ref);
	//Check fields
	if($name=='' || $email=='')
	{
		echo 'Please fill in all fields.';
		exit;
	}

	//Subscribe
	$postdata = http_build_query(
	    array(
	    'name' => $name,
	    'email' => $email,
	    'list' => $list,
	    'boolean' => 'true'
	    )
	);
	$opts = array('http' => array('method'  => 'POST', 'header'  => 'Content-type: application/x-www-form-urlencoded', 'content' => $postdata));
	$context  = stream_context_create($opts);
	$result = file_get_contents($your_installation_url.'/subscribe', false, $context);

	//check result and redirect
	if($result)
		{
		header("Location: $ref");
		}
	else
		//header("Location: $fail_url");
?>

Hope this helps.