To do this, you will need to determine the ERD (effective rim diameter) of your rim. From what I can tell, Vanmoof uses custom rims meaning this measurement needs to be taken manually. I used a few different methods to measure it and then averaged my results.

This park tool article gives one way of measuring ERD and this Sheldon Brown article gives 2 more ways.

You will also need the dimensions of your hub. You can typically find these in the technical manual. I found the technical manual by googling around. If you are using an enviolo hub this is the manual for all of their hubs

The last thing you will need is what cross pattern is appropriate for your hub given the rim size. For the enviolo hub it is 2 cross. I found this in the hub's manual as well.

After you have your ERD you can enter it into this website along with the dimensions of your hub and the cross pattern you intend to use. It will calculate what length spokes you need.