It depends on the stations you are travelling from and to.

On most stations, you will be able to take the train without any assistance. But on some stations, there is a difference in height, or a large gap, between the train and the platform. This will cause that you will need assistance. For that reason, I would suggest you to always request assistance to make sure you can travel without any problems. 

Between which stations do you travel? Maybe someone on this community can share their experiences on your route. 

Hello,
I followed the registration procedure and have a NS account
I need wheelchair assistance from Schiphol to Assen and back, I had no problems buying tickets online
Unfortunately, to book the wheelchair the procedure is interrupted because it does not accept my Italian address, can I book the wheelchair by email or by other methods other than telephone?

Hi Flying Dex and welcome to the NS Community, 
I am sorry to read that you can't create a My NS-account. This is a known issue. If you have Twitter, please send NS_Online a message so they can help you directly to create a My NS-account. It is also possible to book this assistance by phone on 030-2357822.