There is no application form for open work permit. You can use a change of status form for a worker, see here: http://www.cic.gc.ca/english/information/applications/extend-worker.asp
If your common law is currently not on a work permit, I suggest you answer question 3 on page 1 with "new employer", question 1a on page 3 with "open work permit" and I suggest you also write in 1b "open work permit requested upon first stage approval".
