<!-- BEGIN COPY / PASTE -->
    // Add a custom field to the registration form
    function my_custom_register_form() {
      ?>
      <p>
        <label for="custom_field"><?php _e('Custom Field','textdomain') ?><br />
        <input type="text" name="custom_field" id="custom_field" class="input" value="<?php echo esc_attr( wp_unslash( $_POST['custom_field'] ?? '' ) ); ?>" size="25" /></label>
      </p>
      <?php
    }
    add_action('register_form', 'my_custom_register_form');

    // Save the custom field data
    function my_custom_user_register($user_id) {
      if (isset($_POST['custom_field'])) {
        update_user_meta($user_id, 'custom_field', sanitize_text_field($_POST['custom_field']));
      }
    }
    add_action('user_register', 'my_custom_user_register');
    <!-- END COPY / PASTE -->