Posting requires an API token that is (for now) manually granted by the admins, and save when it's required the tokens will only be granted to bot accounts (it is not possible to login into a bot account from the site). If you need an API token just create a ticket on Phabricator explaining what are you going to use it for.
Sadly it's only posting, for now. Commenting goes next and i got a long checklist. As always it's all documented on the API page.
Interesting. What purposes would there be for posting directly from the API?