In class F the clearance is advisory only. Assuming I had filed a Z flight plan I guess I'd simply call ahead of the transition point and say something like xxxx 3 miles to {whatever point you'd filed} request IFR clearance. But you could just still fly your flight plan without a clearance... The alternative from the pilots viewpoint is to get the clearance on the ground if the transition is right after departure. Then you can take off with a squawk and a frequency and a controller ready to talk to you