In your preferred programming language all you need to do is generate a GET request like this:
If you want to send to multiple numbers, use a comma delimited list. Here is an example:
Beware that, due to limitations on the HTTP protocol, sending to multiple numbers is limited to 500 recipients. So, if you want to send the same SMS to more than 500 recipients, e.g. 750 recipients, you'll have to send it in two batches: 500 + 250.