@David H 

it should be just {{ event.extra.checkout_url }}&discount={% coupon_code 'CouponName' %}

www.mysite.com/ is already a part of the {{ event.extra.checkout_url }} so you don’t need to add that in front

Thanks Bobi! This helps a lot.

I found the instruction on the Klaviyo site confusing when it said “add to the end of your site link.” Actually, I’m still not sure what they mean. :)

If you are sending Abandoned Cart emails (triggered by the Checkout Started event) and would like to link a customer to their abandoned cart with a discount code added, add the following to the end of your site link:

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

It actually should be: {{ event.extra.checkout_url|default:organization.url }}?discount={% coupon_code 'CouponName' %}

used a ? instead of & before discount=