Continuing from the example in Your First Message Template, you can send your transactional message in curl with the following.
curl -X POST https://txnhog.co/txn/api/your_message_key -H "Content-Type: application/x-www-form-urlencoded" -d "apiKey=sdkrjwhaldkgh3pwdfohawp4lyu&name=Shawn%20Spencer&phone=2085559988"