The app says that I need a sticker.

No sticker is needed in Belgium.

In some cities, a sticker under your windshield is required to inform parking officers that you’re using the EasyPark app. If a sticker is mandatory in your area, the app will notify you.

How to get a sticker?
You can order a sticker at any time from the app:
Account > Order Sticker > Order Sticker
The sticker will be mailed.

Waiting for your sticker?
If you haven’t received it yet, you can temporarily use a handwritten note saying:
"I’m parking with an app - EasyPark."