Opencart: v2.x & v3.x
You have to create an account here https://www.zendesk.com/register/chat/?source=zendesk_chat to get a PROPERTY ID for your domain.
Edit the file knns_zendesk_formerly_zopim.ocmod.xml
and replace ADD-YOUR-PROPERTY-ID-HERE in line 14 with your PROPERTY ID.
Upload the file knns_zendesk_formerly_zopim.ocmod.xml
inside the folder /system/
.