To volunteer as a marshal for a MotoGP event there is no direct FIM contact that will accept you as a marshal for a particular Grand Prix. As far as I am aware each country that hosts a MotoGP race is responsible for their own marshals. The country ASN is probably FIM certified and therefore can train their recruits to the FIM standards, but again the recruitment process happens on the local level.
So how do you go about registering?
There are a few options however you will have to do some research. The most obvious option is to contact the event organizer. Typically, each event is listed on the motogp.com The web site is well advertised by all news outlets and anyone can theoretically register. There is obviously no guarantee of being accepted.
The other option is to become a member of a marshaling club or organization. For example in the US being a part of USAC or SCCA allows you to get the necessary training to qualify as a MotoGP marshal. So that when registration opens, and a particular organization is selected to provide marshals for that event, you will be more likely to be accepted as a member of the club.
The final option is to contact the circuit where the event takes place. For example when I wanted to marshal the Malaysian GP, the only contacts I could find were ones that were employed by Sepang International Circuit. So it was an obvious choice of first contact. You may go as far as asking for a job with the circuit so that when the big event comes around you have the possibility of working there as an employee. This is the case with many Circuit of the Americas marshals whom were specifically employed by the track to work various track rental and event days, and who were also given an opportunity to work major events when the majority of marshals there are volunteers.
Best of luck and if you have pertinent information to share, please leave a comment below!