Just to make sure I've understood correctly: You have everything configured on the FCM side, now you want to send messages to your devices. Is that it?
If this is the case, and you want to do it the hard way, I think you need to look at the HTTP or XMPP API. I haven't tried it before, so I don't know how easy or hard it is...
But if you want to send notifications, why not use the oneSignal plugin? Doesn't it do what you need?
Cheers,
R