Here are the steps:
1. Go to your account page by clicking your account name in the upper right then click the drop down and select "Create Channel".
2. On the next page, select "External API".
3. Here you can configure how you want TextIt to send you new messages that need to be sent. TextIt can send messages using a standard GET, POST or JSON body, and you can configure the parameters as you wish. Select the type of URN you will be using (likely Phone Number) and fill out the other information detailing how you want TextIt to receive incoming messages.
4. Once you submit this form, TextIt will provide a URL for you to send incoming messages. TextIt expects messages to be delivered using a POST to the URL provided with the following parameters:
text: The text of the message, utf-8 encoded
from: The number which sent the message, this should be fully qualified with country code, ex: +12065551212
date (optional): The ISO 8601 date when the message was sent, this field is optional.
TextIt will respond with a 200 status code if the message is accepted, otherwise 400 if there is an error in your submission with more details as to why.
5. That's it! TextIt will respond to incoming and outgoing messages for your channels as it would for any other. If you experience problems sending outgoing channels with your external channel you can check the sending log from your channel's page to get a detailed log of those requests.