Passing variables to product page after action "add-to-cart"

[The50]

Hello, I need to send specific variable back to product.tpl template file only after the function "add-to-cart", which as I understand refers to CartController.php controller. At the moment there is a whole page reload enabled after adding the product, so I would just need to somehow send some parameter to template. Which function should I use (if it's on Cart.php controller file) in which file? 

I tried adding the variable on initContent() function in CartController.php but the variable is still not available on product.tpl file. 

As I understand the problem here is that CartController.php doesn't render the product page, it only adds the product to the cart, but how can I find the redirect after add-to-cart action? I could pass some variables on that redirection and it should do the work.

Edited April 13, 2018 by The50 (see edit history)