Use POST /api/v2/conversations/messages to send an outbound SMS message. There's only a couple properties in the request, and the documentation on them is pretty straightforward. Let me know if you have specific questions.
I saw, but it seems that it's telling me that my user is not part of the queue I'm trying to send from, how can I find my user when using client credentials?
Okay I see, and are those token only time limited, meaning I need user interaction again once in a while in order to allow my servers to send sms message and do activities on behalf of a user?
You can't create conversations of any type without a user. The user must log in to your app to authorize it the first time, and every time after that when the token expires. There is no option for an unattended server application to create outbound conversations (other than predictive/power/agentless outbound voice campaigns). You can request new product features at https://purecloud.ideas.aha.io/ideas.
Do the access code expire? Or only the access token expires?
Can I just use the same access code again and again to generate new tokens when one expires?
Access codes are for one-time use. Access tokens expire based on how you've chosen to configure your oauth client. As mentioned before, when the token expires, the user must re-authorize; there is no way to authorize an app once and have it stay logged in forever, or even for a long time. Access tokens have a max lifetime of 48 hours.
If you're trying to build an unattended outbound SMS campaign, you'd be better served integrating with a 3rd party SMS service that is built for bulk outbound messaging. SMS in PureCloud is currently only intended for inbound/outbound conversations with an agent and comes with caveats that will make an unattended or bulk solution nearly impossible.
You'd be integrating the SMS provider into your app, not into PureCloud directly. If you want to use PureCloud APIs in the app also, that's possible. We don't generally make recommendations for 3rd party services, especially paid ones. If you search for something like "bulk SMS provider" you'll find plenty of results.