If you host it on the server it's easy. And FMChat already exists:
If you're not hosting with a server, then there's no easy way except using the Open URL script step paired with something like the sms: URL scheme to send the message via the native messages app.
Which are you wanting to do?
- send an iMessage by interacting with the Message app (not aware of Message app interactivity in Go)
- send a message via FileMaker to be read in FileMaker (requires message table, and should use security in FileMaker to determine who can read which message records. May also require onTimer script running to periodically check for received messages.)
- send email (fully supported Go interaction with Mail app, but visibility of recd emails to recipient depends on their own app settings)