PowerBi - Getting 404 but works in Postman

I have some issues with Powerbi which is driving me insane.

Essentially, I have an API endpoint which accepts a JSON body:

{key1:"value1",key2:"value2"}

It is a GET request on an endpoint: https://website.com/api/v1/endpoint

When making the GET request in Postman, everything works fine, and it outputs the response with HTTP 200 OK. In PowerBi (blank query) however, it is returning a 404. I did a console.log on the endpoint, it outputs with Postman, but doesn't output when the same request is made from PowerBi (making me believe its not even reaching the URL).

The Postman body looks like this:

In PowerBi, I am doing this:

= let url = "https://website.com/api/v1/endpoint",
    body = "{""key1"": ""value1"",""key2"": ""value2""}",
    header = [#"Content-Type"= "application/json"],
    response = Web.Contents(url, [Content=Text.ToBinary(body),Headers = header]), 
    Source = Json.Document(response)

in
    Source

I am getting this as a response:

What is strange, is if I changed the PowerBi Text.ToBinary(body) to Json.FromValue(body), it does reach the endpoint and I can see in the console.log output, the following:

SyntaxError: Unexpected token " in JSON at position 0
    at JSON.parse (<anonymous>)

PowerBi also returns this as an error (note this time it is a 400 and not a 404):

Any help would be massively appreciated!

The response from the API would return a JSON object which I would then parse into PowerBi as a dataset.