functions.php

<?php

 /**
  * WPForms, eBook Download URL
  *
  * @author Bill Erickson
  * @see http://www.billerickson.net/code/wpforms-ebook-download-url
  */
 function ea_wpforms_ebook_download_url( $string, $form_data = array(), $fields = array(), $entry_id = false ) {

     if( ! $entry_id )
        return;

    $download = get_post_meta( get_the_ID(), 'ea_wpforms_ebook_download_url', true );
    if( $download )
        $string = str_replace( '{download}', wp_get_attachment_url( $download ), $string );

    return $string;
 }
 add_filter( 'wpforms_process_smart_tags', 'ea_wpforms_ebook_download_url', 20, 4 );