How to test Easypaisa Sandbox API?

Question

I am trying Easypaisa sandbox API to implement bill payment from this site:

API:

https://api.eu-de.apiconnect.appdomain.cloud/easypaisaapigw-telenorbankpk-tmbdev/dev-catalog

path: 

POST /GetBillCompanies

But it is throwing error:

httpCode: '401',
httpMessage: 'Unauthorized',
moreInformation: 'Invalid client id or secret.'

Although I am providing a valid client id and secret. I tried this on postman and in the node.js program.