At the moment it is not possible, and it is also unknown when it will work.
From the beta reports:
xbx wrote: ↑13 Mar 2017, 18:48
hum.. this flag is from turbo for collisions in splitscreen mode not for network play. need to sort this out.
we'll give intructions on how to create trackmania titles with collision at later time - there are some issues to sort out first.