Trying to add a gateway via Fiddler, Postman or Insomnia

I'm just learning the API and am on the Gateway page

https://docs.spreedly.com/basics/gateway/

I can get the example to work using Curl - as shown on the page - but I can't get it to work with Fiddler, Insomnia or Postman.

With Postman  I use basic authentication to pass the username and password but I get the error

{
    "errors": [
        {
            "key": "errors.gateway_required",
            "message": "You must specify a 'gateway' parameter."
        }
    ]
}

I did see on the help site someone had the same problem and the suggestion was to use Insomnia - so I tried with Insomnia and got the error

{
  "errors": [
    {
      "key": "errors.access_denied",
      "message": "Unable to authenticate using the given environment_key and access_token.  Please check your credentials."
    }
  ]
}

But I put in exactly the same values which worked for Curl and got the "gateway" error in Postman.

Finally in Fiddler composer  I get back

HTTP/1.1 502 Fiddler - Connection Failed
Date: Sat, 13 Nov 2021 17:15:34 GMT
Content-Type: text/html; charset=UTF-8
Connection: close
Cache-Control: no-cache, must-revalidate
Timestamp: 17:15:34.989

[Fiddler] The connection to 'core.spreedly.com' failed.  <br />ystem.Security.SecurityException Failed to negotiate HTTPS connection with server.fiddler.network.https&gt; HTTPS handshake to core.spreedly.com (for #7098) failed. System.Security.Authentication.AuthenticationException A call to SSPI failed, see inner exception. &lt; The function requested is not supported

Win32 (SChannel) Native Error Code: 0x80090302

What am I doing wrong?