If I understand this correctly, eastbound flights do get their OCA clearance by Canadian ACCs. The fact that this was translated 1:1 to CPDLC services might be a Vatsimism but even if it were Gander or Shanwick directly, the nature of Hoppie obviously forbids just one logon for several controllers. So if more than one OCA controller or delivery is present, the fixed logon format won't work anymore. Manually editing the station would therefore make sense in any case. Maybe this can be reconsidered?
17.8.3 Canada: https://www.icao.int/WACAF/Documents/Meetings/2019/SAT24/SAT 24 WP 17e _APP E_NAT Doc 007 (EN) - Edition V.2019-2_eff.pdf