Discoverability is an important part of the TMS API set. Querying a known, fixed endpoint returns a list of links that describe where additional resources can be located. Performing a GET to returns a list of services that your account is authorized to use. If your account is enabled for sending SMS messages, you would have received a link to the sms message resource:

    "_links": {
      "self": "/",
      "keywords": "/keywords",
      "command_types": "/command_types",
      "inbound_sms_messages": "/inbound/sms",
      "sms_messages": "/messages/sms",
      "email_messages": "/messages/email",
      "voice_messages": "/messages/voice"