The HiPay test platform is located at https://test-professional.hipay.com/
Try to login with your main account credentials. if that does not work, contact HiPay via https://support.hipay.com/hc/en-us to request a test account.
Please remember that when you use the test platform, you should update API credentials, website ID & environment in the gateway settings with the ones from your test platform.
If you use the test platform, you should see a sandbox notice on the top of the screen:
If you use Belgium/Belfius as payment method, you can simulate a complete purchase from beginning to end:
The test payment will show up at https://test-professional.hipay.com/.
Other test card numbers can be found here: https://support.hipay.com/hc/en-us/articles/213882649-How-can-I-test-payment-methods-
Please note that the HiPay test platform is not available from time to time, and that callbacks are sometimes disabled, or take a while to complete, which means the invoice will not be marked as paid during that time.