Having just read the OP's question again I see he was talking about
IFR flight plans.
As welkyboy says, there is normally nothing to do. It does it all for you. The FP gets sent to two Eurocontrol addresses and they are always the same.
The additional addressing is used mostly with flight plans which have a VFR component, either formally as in Y or Z flight plans in which case EuroFPL automatically addresses the dest or dep airport, but does not address the FIR (not sure that's needed but it might be), or informally as in a farm strip I-plan departure where one may be asking a nearby big airport (CAS owner) for the IFR clearance (and then you have to address that airport manually because EuroFPL has no way of knowing that kind of stuff).
IFR Eurocontrol route development is another topic
Start by playing with
FlightPlanPro.