From the User Guide
20.3 Uncontrolled Airports
Should you find yourself departing from an uncontrolled airport, without Clearance, Tower or Ground control facilities you could be excused for asking 'how on earth do obtain IFR clearance in such a situation'?
New in PF3 version 3.2.0 we have introduced support for obtaining IFR clearance from FSS. Here's how it works...
First you need to tune to FSS and hit hotkey #0, this is your 'wake up' call to FSS... there could be a delay of up to 30 seconds so be patient, they're not ATC controllers. Once they put their coffee down they will ask you to 'go ahead' at which point, if you're using VCP mode 1, otherwise it will be automatic, you can either hit the 'Roger' hotkey or hotkey #0 again.
On doing so your VCP will provide the necessary details regarding your flight plan to FSS, after which they will clear you as filed, give you an initial altitude and also the frequency for Departure, which of course you should contact asap after takeoff. Obviously this new feature is only available when your are at your departure airport.