Unable to send SMS via http2sms: Missing session parameter

I am trying to send an SMS using via a URL / http2sms and are receiving the following error message(s) upon making a request:
_KO_
_Missing session parameter._

Documentation / guide followed:
https://docs.ovh.com/ie/en/sms/send_sms_messages_via_url_-_http2sms/

If you could please advise on what appears to be missing from my URL request and update the appropriate documentation accordingly that would be appreciated.

Kind regards,
Aaron.