Depending on how you’re sending messages, you have a few options:
From an Android phone, Tasker is a good option. It’s not the easiest program to use, however it is very powerful and allows for some crazy automations on your phone in general. A script can be created that will send an SMS via your standard SMS application.
While I don’t remember the specifics, I believe there are ways to get Tasker scripts to trigger from an external source.
Purely from a computer, you could look at using a VOIP service that offers messaging. A non-exhaustive list includes: