This first step takes place in the Trustpilot interface. You must go to the back office and access the API management section. Here is a direct link to this part: https://businessapp.b2b.trustpilot.com/#/applications/
Here is a screenshot of this interface:
You must then create an application to take advantage of Trustpilot REST APIs. You will access to an API Key and an associated secret code, which will be useful during the next steps of the activation process.