Steps to reproduce:
see that there is no hint how to add items to templates. There is one info that templateid is ro-variable. There should be a hint that you have to use hostid param to add items to templates. Also the description of hostid in the docs could provide something like
"ID of the host or template that the item belongs to."
Furthermode interfaceid is not required either when you add items to templates. It is only required when adding items to hosts.