getting PV as null when accessing through external system(Appian) by using API key in integration

Hello,

I have a requirement to get process variables in http response when accessing from external system (Appian) by using API key in integration. When using Authentication as basic, I was able to achieve the desirable output.

When using an API key with header name/ Appian-API-Key/ x-API-Key the process variables in http response output is null. The process model started and finished successfully While using API key. 

[{"pp":{"id":537711392,"name":"","initiator":{"id":""},"timeZone":"","starred":false,"parentName":""},"pm":{"uuid":"","name":"","description":"","version":"","creator":{"id":""},"timeZone":"","starred":false,"parentName":""},"pv":{}}]

Is there anyway to get PV when using API key in integration?

Thank you.