links can not be modified

Hello, I installed the product WHMCS 5.3, I set all the required parameters and created products.
I want to sell products with two currencies, Swiss Franc CHF and Euro €.
I inserted the two prices in the products, but I can not change the link of the shopping cart.
In the documents I followed the indication:

"Specifying Currency for a link

You can link to the order form with a specific currency by simulating what the form shown for currency selection does - eg. cart.php? currency = 1 cart.php? currency = 2 "

Proceeding as a guide, inserting the link as an example:
"whmcs.miodominio.ch/whmcs/cart.php?currency=1" or "whmcs.miodominio.ch/whmcs/cart.php? currency=2"

The currency is not updated it changed.

How can I fix?

thanks