If you are an existing patient, you can use the eHealth service of the Belgian government to access mijnsintlucas. You can authenticate with the itsme app on your mobile phone or with an eID card reader connected to your computer.
Alternatively, you can also use the unique box code you can request at the hospital. You will need a mobile phone to verify your identity via SMS.
Unfortunately, you can not access mijnsintlucas if you are not currently a known, registered patient.