title: ACF relationship field with add_post_meta
body: <p>i try to create a wordpress post by prog.</p>
  
  <p>First, i use <code>WP_insert_post</code> to create the post, and it return me the id of the post just created.</p>
  
  <p>After that, i want to add information to the custom field on the post, so i use the ID and add_post_meta like that:</p>
  
  <pre><code>add_post_meta( $post_id, 'info', 'This is my value' );
  </code></pre>
  
  <p>For a WYSIWUG Editor or textarea or a textfield ACF, it working fine.</p>
  
  <p>But i also have a relationship ACF and, i try with <code>add_post_meta</code> like for the textarea, but it not working!</p>
  
  <p>How can i make <code>add_post_meta</code> work with relationship ACF?</p>