Getting messages and post activity

Unfortunately, instant-messaging and activity endpoints are not supported by the TumblrSharp library. Although the endpoints exist, they are restricted to official Tumblr consumer keys and consumer secrets. Although it is possible to extract the consumer_key and consumer_secret from the official Tumblr Android app (as the Ouga Windows app has successfully done), Tumblr will give you either a 404 or 401 error when trying to access the resource with your own app keys.

It is technically possible, but TumblrSharp will not be implementing this functionality, as it isn't future-proof, and distributing Tumblr's API keys in an open-source project isn't respectful to their service.

It is unclear when these endpoints will be opened to the public.

Doing it yourself

If you really want to get this working, it's possible with the help of a few tools. Use mitmproxy to capture HTTP traffic between your phone running the application and Tumblr servers. You can use this to discover undocumented endpoints. You'll also need to extract Tumblr's official API keys from the app itself.