Sending to a peer:

sip.conf

[user2]
type=peer
host=127.0.0.1
port=5062
context=not_default
username=user2
secret=blah

extensions.conf:

exten => user,1,NoOp()
exten => user,n,MessageSend(sip:user2)

Sending to a URI:

exten => test,1,Answer()
exten => test,n,Set(MESSAGE(body)=I like waffles.)
exten => test,n,MessageSend(sip:user@127.0.0.1:5062)