asterisk users - Oct 2018 - messagesend to SIP peer in sip.conf (or otherwise authenticated)

Hi,

I want to be able to send SIP SIMPLE messages via/to my VOIP provider
but in trying to do so with MessageSend() I am getting 401 errors back
from them, unsurprisingly.  They want such messages from me
authenticated with my account just as they would for SIP voice calls.

For voice calls, of course I use Dial(SIP/remote at peer) where remote is
the phone number I want to call and peer is my SIP provider's entry
from my sip.conf complete with account and password.

So how do I achieve the same thing with MessageSend()?  When I try to
use the same kind of recipient string I get an error:

[Oct  1 09:48:40] WARNING[3286][C-000013ab]: message.c:1232 msg_send_exec: No
message technology 'SIP/sms-number at sip-peer' found.

Using MessageSend() with raw URLs (sip:
sms-number at sip-provider.example.com) is accepted by Asterisk but it
provides no authentication to the MESSAGE command sent to sip-
provider.example.com so is rejected.

So how can I, preferably, use my existing sip.conf entries to send SIP
SIMPLE messages, or at least provide some kind of authentication
otherwise with MessageSend()?

Cheers,
b.

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