add_action('user_register', 'send_new_user', 10, 1);

function send_new_user ($user_id){

    $new_user = get_userdata($user_id);
    $useremail = $new_user -> user_email;

    $url = 'https://api.capsulecrm.com/api/v2/parties';
    $body = array(
                'firstName' => 'WhyWontYouWork',
                'email' => $useremail

    );

    $args = array(
        'method' => 'POST',
        'timeout' => 45,
        'redirection' => 5,
        'httpversion' => '1.0',
        'sslverify' => false,
        'blocking' => false,
        'headers' => array(
            'Authorization' => 'Bearer {private token goes here!!!!}',
            'Content-Type' => 'application/json',
            'Accept' => 'application/json',
        ),
        'body' => json_encode($body),
        'cookies' => array()
    );

    $request = wp_remote_post ($url, $args);
};