Where does the form action go?

I have a very simple form from an old site we are converting to WordPress. The form only asks for an email address and displays a Sign Up button.

I cannot figure out where to add the form action. Where in the heck does it go?

If I knew where to put it this is the string I would add this:
"http://app.streamsend.com/public/SDBo/V8c/subscribe"

Thanks David.

I think that the second line, below, is edited correctly because the form I want to use has an ID of 2.

If I only have an email address in my form to send to a third party service I would add this to the functions.php as shown below. I'm only guessing because I don't know how to determine the value I found in the braces example: [1] or the name of the email field (I don't see anywhere in the form that tells me that). Even if I'm right about where to put the path I don't know what to do in the array. I probably don't need the array part since it is one value but lets not worry about that.

I would also like to know what the 10 and 2 stand for in the second line of the code below.

I feel like I'm close to the proper solution I hope you can help. Thanks.

?php
add_action('gform_after_submission_2', 'post_to_third_party', 10, 2);
function post_to_third_party($entry, $form) {

    $post_url = 'http://app.streamsend.com/public/SDBo/V8c/subscribe';
    $body = array(
        'email' => $entry['1']
        );

    $request = new WP_Http();
    $response = $request->post($post_url, array('body' => $body));

}
?>