title: Add a field to Woocommerce registration form and in admin edit user
body: <p>In WooCommerce, I have added extra field to account registration form: </p>
  
  <pre><code>// 1. ADD FIELDS
  add_action( 'woocommerce_register_form_start', 'add_woo_account_registration_field' );
  function add_woo_account_registration_field() {
      ?&gt;
  
      &lt;p class="form-row form-row-first"&gt;
      &lt;label for="reg_billing_account_number"&gt;&lt;?php _e( 'Ship to/ Account number', 'woocommerce' ); ?&gt; &lt;span class="required"&gt;*&lt;/span&gt;&lt;/label&gt;
      &lt;input type="text" class="input-text" name="billing_account_number" id="reg_billing_account_number" value="&lt;?php if ( ! empty( $_POST['billing_account_number'] ) ) esc_attr_e( $_POST['billing_account_number'] ); ?&gt;" /&gt;
      &lt;/p&gt;
  
      &lt;div class="clear"&gt;&lt;/div&gt;
  
      &lt;?php
  }
  
  // 2. VALIDATE FIELDS
  add_filter( 'woocommerce_registration_errors', 'validate_woo_account_fields', 10 );
  function validate_woo_account_fields( $errors, $username, $email ) {
      if ( isset( $_POST['billing_account_name'] ) &amp;&amp; empty( $_POST['billing_account_number'] ) ) {
          $errors-&gt;add( 'billing_account_number_error', __( '&lt;strong&gt;Error&lt;/strong&gt;: account number is required!', 'woocommerce' ) );
          $fields['billing_account_number']['maxlength'] = 6;
      }
      return $errors;
  }
  
  // 3. SAVE FIELDS
  add_action( 'woocommerce_created_customer', 'save_woo_account_fields' );
  
  function save_woo_account_fields( $customer_id ) {
      if ( isset( $_POST['billing_account_number'] ) ) {
          update_user_meta( $customer_id, 'billing_account_number', sanitize_text_field( $_POST['billing_account_number'] ) );
          update_user_meta( $customer_id, 'account_number', sanitize_text_field($_POST['billing_account_number']) );
      }
  }
  </code></pre>
  
  <p>But <strong>the field validation doesn't work</strong> as the user still can register without filling it. <br>
  What I am doing wrong?</p>
  
  <p>Also when customer register I would like to see this field on the admin user page.</p>
  
  <p>Any help is appreciated.</p>