How to pull data from Web API into PowerBI using Authorization header

So I've found the solution and it was due to a PowerBI expecting and requiring a content in the API request. Not sure why this is required by PowerBI but this is the magic step I was missing. Full simplified example below:

let
    accesstoken = "token " & #"API",
    url=Text.Combine({#"URL", "/api/tokens/authenticate"}),    
    Source = Json.Document( Web.Contents(url, [ Headers = [#"Authorization" = accesstoken], Content = Text.ToBinary("grant_type=client_credentials") ]))
in 
	Source

The Content of "grant_type=client_credentials" seems to be required for PowerBI when handling the authentication yourself in the query.