In your preferred programming language all you need to do is generate a GET request like this:
https://api.textmarketer.co.uk/gateway/?username=u&password=p&to=4477777777&message=test+message&orig=my_script
If you want to send to multiple numbers, use a comma delimited list. Here is an example:
https://api.textmarketer.co.uk/gateway/?username=u&password=p&to=4477777777,4477777778,4477777779&message=test+message&orig=my_script
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.
Here's a summary of the different variables you need.
*to view your scheduled sends, log in to the customer system and go to “reports” =>“sent sms”.
You can also cancel or amend the queued sms.