How can I use other fieldname for "key" required field in jwt?

[NPhGit]

We have existing system that use tyk gateway. The auth system of us is in separated system of other team. We need to use apisix as our new gateway but we can't change the structure of existing jwt. It has 'sub' as user identifier field and 'apim_po' as policy identifier (customizable in tyk). Apisix require jwt to have 'key' field as identifier. How can I change this "key" fieldname to something else?