Authentication via body with different URL from Base?

Not sure if I’m not readying the documentation right but I am having a problem configuring the HTTP Requests plugin.

First up, some APIs require authentication information to be passed in the Body instead of the Header. I see that the Body is available in the Actions but not in the connection step. Is there a way to authenticate a connection with the Body?

Second, the Base URL is provided in the connection step. Actions then require Routes which are concatenated with the Base. But what if the authentication requires a separate URL from what will be the Base of all other requests?