Add a datepicker field in the product front page to choose the delivery date

[Eliott_Crmx]

Hello,
I need your help to understand how to retrieve the data from the product page form (product.tpl) sent by the 'add to cart' button (product-add-to-cart.tpl include).
Indeed, I would like to retrieve the value of a datepicker field, added below the input dedicated to the quantity of the product page,
in order to save it in the ps_cart_product table.
Despite my research in the cart class and its controller, I can't do it. How to get the value of an input in the product front page ?