It is not clear what you want.
If you want to add this notice to all outgoing texts you send, you need to install a texting app that has a "signature" feature. This is very common with third-party apps.
If you want to have this notice to out as an auto-response to every text you receive, you would need to find an app with that feature. This would be uncommon.
Do you really have a privacy risk that others need to be aware of?