Set up a connection to Worldline/Ingenico through Datatrans' PCI Proxy

Question

I created an account on Datatrans' PCI Proxy. The goal is to set up a connection to create/authorize a payment on Worldline/Ingenico.

As supported test endpoint PCI Proxy suggests this one: https://eu.sandbox.api-ingenico.com

I created an account on Worldline's Sandbox using Europe as a location to test a REST API. The problem is that I cannot create an authorisation key in order to succeed in an API call.

I'm following these instructions to make an API call on Postman https://apireference.connect.worldline-solutions.com/s2sapi/v1/en_US/java/authentication.html?paymentPlatform=ALL I'm creating an authorisation key using HMAC-SHA256 encryption but every attempt resulted in an error code:

 "id" : "MISSING_OR_INVALID_AUTHORIZATION",
  "category" : "CONNECT_PLATFORM_ERROR",
  "message" : "MISSING_OR_INVALID_AUTHORIZATION",
  "httpStatusCode" : 403

Is there something I am missing?