If the flight is entirely within Class F airspace, why do you need a clearance and from whom? It's uncontrolled airspace, and doesn't even have to have FIS. However, presuming an IFR flight plan is in the system and some form of FIS/ATC is available, then I would start with a variant of a position report to the relevant FIS/ATC plus either a notification that I was now IFR & request whatever FIS is available, or a request for an IFR clearance if the airspace category will change to one that requires a clearance ie:
'Name of FIS/ATC station'
Callsign
Position (+ ATA if non radar)
Altitude
+
IFR (destination, + via xyz if thought necessary), request Radar Advisory Service (or whatever); or
Request IFR clearance (destination).