You check in in the Workit app, from 30 minutes before the start time until 30 minutes after the end of your booked workout. The app uses GPS to check that you're near the location and then shows a pop-up with a check-in button. No pop-up? Go to Bookings and tap Check in.
Important: give the app access to your GPS location, otherwise you can't check in and you risk a no-show fee. You can also turn on the reminder to get a notification automatically. You'll find more information at workit.nl/gps-check-in. You don't need to check in at walk-in partners.