Adding a custom Wordpress shortcode for building a dynamic dropdown of post type titles with Contact Form 7

custom_post_type_cf7_dropdown.php

wpcf7_add_shortcode('postdropdown', 'createbox', true);
function createbox(){
  global $post;
  extract( shortcode_atts( array( 'post_type' => 'some_post_type',), $atts ) );
  $args = array('post_type' => $post_type );
  $myposts = get_posts( $args );
  $output = "<select name='postType' id='postType' onchange='document.getElementById(\"postType\").value=this.value;'><option></option>";
  foreach ( $myposts as $post ) : setup_postdata($post);
    $title = get_the_title();
    $output .= "<option value='$title'> $title </option>";

  endforeach;
  $output .= "</select>";
  return $output;
}

Hello,

How would I make this select field required? The standard way does not work: [postdropdown* jobPosition post_type="job-opening"]

Unfortunately, possibly because of changes in CF7, this code no longer includes the output of the field in the mail