First off, in Postman you might have to turn off SSL verification:

The next step is authenticate. This is done via a POST call to generate an API token.

The URI is:

The body of the request is your username and password in JSON form:

In Postman:

Make sure to select raw and then JSON as the formatting under body.

Click Send and you will receive the API token:

Now use that to create the session. This is done via another POST call:

With the body being the returned API token:

Add it like so into Postman click send:

If all is good, you will get the username as a response to confirm the session:

Now you can make your REST calls as needed:

For the REST API guide, go to your FlashArray GUI and click Help. It is linked from there.