Payment Initiation Service.

Payment initiation is done using the integrated OAuth2 authentication flow described in NextGenPSD2
This means you first have to initiate a payment using the dedicated endpoint, then redirect the PSU to our
authorisation page following the link present in the response (using PIS:{paymentId} as OAuth2 scope).
After performing a strong customer authentication, the PSU will be redirected to your redirect URI with an
authorisation code that you can then exchange for an access token.

This access token is only needed to cancel a payment on behalf a PSU (this does not require a strong
customer authentication). It is not needed to retrieve the current status of a payment.