I just did my first two North Atlantic crossings last week with a 7x using CPDLC. We could not get the clearance on ground with CMF/ATS in Shannon (did fuel stop). Received the NAT clearance with Shannon oceanic as usual by voice 45min before Take off. ATC advised us to logon CPDLC after Take off, climbing through FL160 not before. CPLDC worked fine within FIR. We crossed via Shanwick FIR - Santa Maria - New York FIR. But Automatic switch over from Shanwick to Santa Maria FIR did not work. We had to log on manually using the FIR AFN Address. BUT, the FIR Identifier on the OFP or Plotting chart was not correct. Advice: use the GOLD Manual and North Atlantic orientation Charts to get the correct AFN Address. E.g. New York FIR is KZNY but for CPDLC it is KZWY. New York gave us early the VHF Frequency to contact and CPDLC disconnected outomatically when entering New York Airspace.