Dynamic checkout code (auto-apply) not working

Hey, 

I wanted to set up my dynamic checkout code - which is not working, this is my code:

{{ event.extra.checkout_url }}&discount={% coupon_code 'WARENKORB10' %}

When I do the “send test” it works, but when I try it on another device with another email it just takes me on the homepage and does not add the discount.

In theory this should work. (I tried replacing & with ? also).

• I set up discounts correctly. As by guide in klaviyo etc.

Any tips how to solve this?

Filip