It seems undocumented, but the requirements to post to the events api are as follows:
description, url, title, and service
If you have all those in your json object and post to the events api /projects/x/events.json it will work.
Question
The github integration with Basecamp adds items to the progress and events section of Basecamp without actually adding a message, calendar, todo, etc. When looking at the api https://github.com/37signals/bcx-api the events are all read-only. I tried doing a post request to /projects/x/events.json but got back a (422) Unprocessable Entity error.
La solution
It seems undocumented, but the requirements to post to the events api are as follows:
description, url, title, and service
If you have all those in your json object and post to the events api /projects/x/events.json it will work.