There's a C API too, but I bet you could use the cli tool in a dhcpcd hook script, although there may be an issue there since the "sending process" would then be part of the dhcpcd service -- but you could write a simple service that just waits for a signal (sent from a dhcpcd hook), then notifies, and have whatever you want started when the network is live depend on that.

Arggg, I'd bet there's a few questions around here that could do with that in an answer.

Having never used systemd-networkd, I'd hope it implements this already in some useful way...