You treat is as if it was any other “Local Server”… only you use the Public IP of wherever your Local Server is setup, and whatever external Port you used for the port forwarding…
I.e. Public IP
XXX.XXX.XXX.XXX and assuming you setup port forwarding as
4321(external) forwarding to
Blynk.begin(auth, ssid, pass, "XXX.XXX.XXX.XXX", 4321); // Use the Public IP and Port of the Local Server location
Use a proper external port number or even keep it the same
8442 - 8442
Server uses ports 8443 (for app), 8442 (for hardware without ssl), 8441 (for hardware with ssl).
Port 9443 (for https, web sockets & admin port).