It is written on CIC as follows
Applicants must submit an Application to Change Conditions, Extend my Stay or Remain in Canada as a Worker form [IMM 5710 (PDF, 447.94 KB)], indicating that they are applying for an open work permit.
I think it is the same form for all kinds of work permit.
Here is the link.
http://www.cic.gc.ca/english/information/applications/extend-worker.asp