Hello, @mohamed aly,
Actually, to add free returns for selected products you have to create multiple single product page layouts in the “Single Product Builder” feature of our theme see image: https://postimg.cc/QVq9BhjX you can find this feature in the theme settings >> WooCommerce >> Single product builder
Once you created a layout you can add this free return using a custom HTML block like this: https://postimg.cc/gnT0R0QP this is the only possible way.
Regards 8Themes Team.