Sharepoint Authentication using REST API - Postman

Question

I'm trying to use the Sharepoint REST API and am stuck at the Authentication Step.

My tenant will not allow the registration of an App because they do not have support for site-specific permissions yet, so I want to request a FedAuth Cookie at runtime.

I am stuck finding the correct parameters for what request to make, in order to generate this cookie.

When following through Developer Tools via the browser, there are about 8 requests passed back & forwards.

Is there an endpoint / series of endpoints where I can pass my username/password, and generate a FedAuth Cookie, using the Sharepoint REST API?

Thanks