Credentials are accessible once you have filled in the sandbox account creation form.
You will then receive an e-mail inviting you to create a password. Once you've entered your password, you can access the test dashboard at the following address: https://test-emoney-services.w-ha.com/dashboard/#/login
This takes you to your home page:
To retrieve your credentials, simply go to the PARTNER tab and click on SECURITY :
You can find the documentation to support you throughout the integration process here