@jeffxy APIs are typically stateless. That means they don’t use things like sessions to persist data between requests. A request should contain everything needed to process that request in isolation.
So, in your case, you need some way of identifying a tenant in each request so yes, in the path (i.e. /companies/{tenant}) would be one method.
Another method, you could issue tenant-specific tokens. Similar to Facebook. I can authenticate against Facebook and get an access token for my own account, but can then also get an access token for a specific Page I’m an admin of and use that access token to interact with Facebook’s Graph API as that particular Page.