When a new text is generated MyAX will send a HTTP POST request to the webhook URL. It has a signature header to verify the integrity/authenticity and some data about the object and the text in the post data body.
HTTP_X_MYAX_SIGNATURE: "sha1=47d5b4154bc4161aaf4279d127f1a52d64895da1"
The checksum is calculated as hmac sha1 hexdigest. The key is your API token. The message is the post data.