ShoppingCart discount with Groups

[Padel]

I want to give a created group a 15% discount on the total of the ordered items. I know that there are problems with this within Prestashop, so here is the question how I should do this and whether there is a special Module for it.

What I am doing now is the following, however, this does not work:

1) I create a separate group and do NOT enter a discount percentage

2) I go to Cart rules and set that only the created group is entitled to a 15% discount.

3) A guest user places an order, but must first register and select a group in the registration form.

4) After a refresh I see that the 15% discount is there, but I want the 15% discount to be right there.

How do I solve this and with which module ??

Please HELP!!