BOGO

 /**

 * @snippet       BOGO

 * @how-to        Get CustomizeWoo.com FREE

 * @sourcecode    https://businessbloomer.com/?p=111758

 * @author        Rodolfo Melogli

 * @compatible    WooCommerce 3.5.4

 * @donate $9     https://businessbloomer.com/bloomer-armada/

 */

 

add_filter( 'woocommerce_add_to_cart_validation', 'bbloomer_bogo', 10, 3 );

  

function bbloomer_bogo( $passed, $product_id, $quantity ) {

   $sku_with_gift = 'sku0001';

   $sku_free_gift = 'sku0002';

   $product = wc_get_product( $product_id );

   $sku_this = $product->get_sku();

    if ( $sku_this == $skuswithgift ) {

       WC()->cart->add_to_cart( wc_get_product_id_by_sku( $sku_free_gift ) );

   }

   return $passed;

}