Add custom field to Relationship field output

acf-functions.php

<?php function my_acf_relationship_result( $html, $post )
{
  // add an image to each result
  $field = get_field('varenummer', $post->ID);

  if( $field )
  {
    $html = '<strong>' . $field . '</strong> - ' . $html;
  }

    return $html;
}

// acf/fields/relationship/result - filter for every field
add_filter('acf/fields/relationship/result', 'my_acf_relationship_result', 10, 2); ?>