For myBSN payment method there aren’t any test data available, but you can see how it works with the payment flow given below.
myBSN Payment Flow
The customer enters his email address, name and phone number.
The customer is shown the details of his payment and proceeds to pay with myBSN.
The customer logs in to his account by entering his Username and completes the payment.
Upon completion of the payment flow, the customer is redirected back to your ReturnURL.