Firstly, you should get your application credentials (application-id, application-secret) from the Email Routee Platform (email.routee.net) and on the Account settings/ API.
Then you should encode the id:secret string to a Base64 string.
Having this string you are now able to exchange your application credentials for an access token by calling the endpoint.
The Authorization header of the request must include the word Basic followed by the encoded string.
Each application is associated with an Account that belongs to the user. Any requests that are made using an access token from an application are the same as if they were made by the associated account.