This is available by using slightly modified URLs that are configured to present the appropriate challenges. You simply introduce 'basic' to the url as follows:
Form Based Authentication:
This is used only by the IBM Connections components themselves when using the APIs from the browser (as they have an appropriate session available to them). Firebug can easily be used to determine requirements, but it should be noted that this means of authentication is subject to change.
OAuth is used to authorize rather than authenticate as technically you have a consumer token that a given user 'authorizes' to act on their behalf in certain circumstances. This is covered separately. The PST requests should be directed at an oauth specific URL: