Hey there @Jaap-CAD2M 
At this time, I’m afraid it is not possible to query custom User fields via the API. In this specific case, it seems like you could query the “name” field instead, but I definitely understand you might have wanted to query a custom field that stores information not available in other user fields.
I’ve passed this as an improvement suggestion to our product team 
That said, I can’t confirm when or whether this would be part of our API at this time.
I hope this helps clarify!
-Alex