do_action ( 'jetpack_subscriptions_form_submission', string $result )

Notes

In the example below, we’ll use the action to trigger a redirection to a specific page as soon as a reader successfully submits their email address in Jetpack’s Subscription form:

function jeherve_custom_sub_redirect_page( $result ) {
    if ( 'success' === $result ) {
        $thanks_page = 'thank-you';
        wp_safe_redirect( $thanks_page );
        exit;
    }
}
add_action( 'jetpack_subscriptions_form_submission', 'jeherve_custom_sub_redirect_page' );