Unfortunately, it is not possible to use the Intercharge app with your Plugsurfing credentials.
Intercharge and Plugsurfing are two different service providers. You will find all charging stations with the Intercharge label also in the Plugsurfing app. All of them are accessible via Plugsurfing. For that you can also use the QR code. The Intercharge app is a service for E-mobility Service Provider not offering a app of their own.