// Hook for adding html to the product edit page under linked products
add_action( 'woocommerce_product_options_related', 'add_linked_custom_product_field' );
function add_linked_custom_product_field() {
  global $product_object; // UGH globals.
  ?>
    <div class="options_group show_if_variable">
      <p class="form-field">
        <label for="customLinkedProdField"><?php esc_html_e( 'Custom Linked Field Products', 'woocommerce' ); ?></label>
        <select class="wc-product-search" multiple="multiple" style="width: 50%;" id="customLinkedProdField" name="customLinkedProdField[]" data-sortable="true" data-placeholder="<?php esc_attr_e( 'Search for a product&hellip;', 'woocommerce' ); ?>" data-action="woocommerce_json_search_products" data-exclude="<?php echo intval( $post->ID ); ?>">
          <?php
          $product_ids = !empty( get_post_meta($product_object->get_id(),'customLinkedProdField', true) ) ? get_post_meta($product_object->get_id(),'customLinkedProdField', true) : array();
          foreach ( $product_ids as $product_id ) {
            $product = wc_get_product( $product_id );
            if ( is_object( $product ) ) {
              echo '<option value="' . esc_attr( $product_id ) . '"' . selected( true, true, false ) . '>' . wp_kses_post( $product->get_formatted_name() ) . '</option>';
            }
          }
          ?>
        </select> <?php echo wc_help_tip( __( 'This lets you choose which products are part of this group.', 'woocommerce' ) ); // WPCS: XSS ok. ?>
      </p>
    </div>
  <?php
}

// Filter for saving custom product data
add_filter( 'save_post_product', 'save_custom_product_options' );
public function save_custom_product_options( $post_ID, $product, $update ) {

  // DO YOUR OWN SANITIZATION HERE!!!! IMPORTANTTT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  $customLinkedProdField = isset( $_POST['customLinkedProdField'] ) ? $_POST['customLinkedProdField'] : array();
  update_post_meta( $post_ID, 'customLinkedProdField', $customLinkedProdField );
}