To send a message from a Drip Workflow, first go to the Drip Messaging page and select Add Message.
From that point you will see the message compose page.

Label your text message to make it easy to find within YetiText. Select the from phone number you wish to send your message from and add the text you would like to send in the message field. You can personalize your message using contact information stored in Drip by clicking the Merge Field button in top left. Additionally you can select a Trackable Link to insert into your message which will allow you to apply a tag to the contact record when that link is clicked by the contact. Using the Manage Media button, you will also be able to attach a media to your message which will automatically change that message from an SMS to an MMS which will allow you to send up to 1600 characters with the media attached. 

As soon as your message is finished, click the Add Message button. 

From that point, you will be taken back to the main Drip Messaging page where you will see the message you created listed by whatever label you gave the message. 

From this point just copy and paste the post URL into a HTTP Post action inside Drip's WorkFlow builder.