Excel Powerquery REST API Call

    url = "https://example.com/oauth/connect/token",
    headers = [#"Content-Type" = "application/json", #"trackingId" = "0a48aa65-dd11-4d03-b166-4b1aa25d5a0f"],
    clientSecret = "someclientSecret", 
    postData = "{""clientId"": ""someclientID"", ""clientSecret"": """&clientSecret&"""}",
    getToken =  Json.Document(Web.Contents(
        url,
            [
                Headers = headers,
                Content = Text.ToBinary(postData)
            ]
    )),
    data = getToken[access_token]