Custom Price in Add-Product Form

[Grafeco]

Hello!

I created a "relative product" in the product page, but the product price of this "relative product" is calculating depends of a product price of the principal product.

So, I need to add the Specific Price of this relative product to the cart.

My form is this:

 

<form action="https://myurl.com/cart" method="post" id="add-to-cart-or-refresh">      
  
  
      <input type="hidden" name="token" value="d296972b7f30e2c19afc34b70eaf45e8">
      <input type="hidden" name="id_product" value="35" id="product_page_product_id">
      <input type="hidden" name="id_customization" value="0">
  	  <input type="hidden" name="qty" value="1" min="1">
  
  
      <input type="hidden" name="customfinalprice" value="12.12">
  
  
         
      <button class="btn btn-primary add-to-cart btn-relacionados" data-button-action="add-to-cart"><i class="icon-Ico_Cart"></i> Add to cart</button>
</form>

 

where name="customfinalprice" is the custom price of the relative product and 12.12 is the value of the custom price.

I attached the imagen of the example (the price of the principal product and the related product are calculated by the size):

 

But when I add the specific product to the cart, the price is not 12.12 €. How can I change this?

 

Thank you!!

Kind Regards.