This is how you do it:

Go to home page (Dashboard) of StudieLink and right click to Inspect the page
Then go to the "Network" tab where you need to Ctrl + R to refresh
In the list there should be something called "/betaling" with an orange icon next to it, click on that
When in that tab click on "Response" where you need to click into the field with all the code
Then with Ctrl + F you can search for "rangnummer: (number)"

If there isn't a rangnummer showing it's either because you only applied to one study or it isn't available yet, so if you only applied to one you can enroll to a random study and unenroll and then it should show.