Saving Cookies from POST Request to Rest API

In my experience, a "we reached the end of the buffer" error indicates that the response isn't in JSON format. It's hard to debug your code since you haven't provided an actual URL, but a few questions:

1. Are you trying to capture the token returned by the response or the cookie? I know of no way to capture the response cookie using `Web.Contents`.
2. Can you provide an actual URL and endpoint to test?
3. If not, can you provide a screenshot of both the working response from the Postman console and the non-working from the Inspector tab in Fiddler?

To help debug, I suggest being more explicit by writing more query steps in your Power Query M code. In other words, each step in your query should contain no more than one Power Query M function. This way, you can step through and try to understand where the failure occurs.

I also suggest creating a record of the contents you're sending in your POST request and using the `Json.FromValue` function to convert the record into a JSON binary format. In your case, I would define `TokenOpts` as a record and then convert that record into a JSON binary format as part of the contents in your POST request.

Here's a sample Power Query M query that illustrates what I'm saying:

let
    TokenUrl = "https://baseurl.com/authentication/sign_in",
    TokenBaseUrl = Text.Combine({Uri.Parts(TokenUrl)[Scheme],"://",Uri.Parts(TokenUrl)[Host]}),
    TokenRelativePath = Uri.Parts(TokenUrl)[Path],
    TokenQuery = Uri.Parts(TokenUrl)[Query],
    TokenOpts = [
        user = "user123",
        password = "pass123"
    ],
    TokenAct = 
        Web.Contents(
            TokenBaseUrl, 
            [
                Headers=[
                    #"Content-Type"="application/json"
                ], 
                RelativePath = TokenRelativePath,
                Query = TokenQuery,
                Content = Json.FromValue(TokenOpts)
            ]
        ),
    jsonResponse = Json.Document(TokenAct)
in
    jsonResponse

My hunch is that the JSON string you've put together is malformed in some way, which is causing the API server to return an error response in some non-JSON format.