WordPress: Use Jetpack's Contact form to protect download links by providing links in form's success message.

wp-jp-downloads-form.php

<?php
/**
 * Use Jetpack's Contact form to provide download links (in success message).
 *
 * Filter the Jetpack Contact Form's message returned after a successful submission.
 * Put download links in a custom field to display in success message.
 *
 * @param string $r_success_message Success message.
 * @return string $r_success_message Success message.
 */
function my_download_form_links( $r_success_message ) {
    $post_id  = get_the_ID();
    $dl_links = get_post_meta( $post_id, 'my_download_links', true );
    $dl_head  = get_post_meta( $post_id, 'my_download_heading', true );
    $heading  = ($dl_head) ? $dl_head : 'Your download links:';
  
    if ( get_post_type( $post_id ) == 'orange' && ! empty( $dl_links ) ) {
        // Add heading to success message.
        $r_success_message  = "<nav class="download-links"><h2>$heading</h2>";
        // Display custom field content (with downbload links).
        $r_success_message .= wpautop( $dl_links ) . '</nav>';
    }
    return $r_success_message;
}
add_filter( 'grunion_contact_form_success_message', 'my_download_form_links', 10, 1 );

/**
 * Add a legend to the form.
 * Filter the HTML of the Jetpack Contact Form.
 *
 * @param string $r Contact Form HTML output.
 * @param string $field_label Field label.
 * @param int|null $id Post ID.
 * @return string $r Contact Form HTML output.
 */
function my_download_form( $r, $field_label, $post_id  ) {
    $dl_links  = get_post_meta( $post_id, 'my_download_links', true );
    $dl_legend = get_post_meta( $post_id, 'my_download_legend', true );
    $legend    = ($dl_legend) ? $dl_legend : 'Please complete form to download files';
  
    if ( get_post_type( $post_id ) === 'my_post_type' && 'My First Field' == $field_label ) {
        $r  = "<legend><h2>$legend</h2></legend>" . $r;
    }
    return $r;
}
add_filter( 'grunion_contact_form_field_html', 'my_download_form', 10, 3 );
?>