title: WooCommerce hook into coupon validation by user meta
body: <p>i'm searching for the hook where i can hook into the coupon validation during the checkout process.</p>
  
  <p>Instead of validation by user's email address i would like to validate the coupon code by an user's meta field.</p>
  
  <p>If the restricted coupon code matches the user meta field the discount is valid, otherwise coupon can't added to cart.</p>
  
  <p>Which hook can i use?</p>