We could not authenticate with the credentials provided

Hello everyone, 

I am trying to get data from an atlassian marketplace REST API with POST request: 
I have the following query: 

let
url = "https://marketplace.atlassian.com/rest/2/vendors/...",
body = "{...}",
token = "username:token",
header = [
#"Content-type" = "application/json",
#"Accept" = "application/json",
#"Authorization" = "Basic"& token],
response = Web.Contents(url, [Headers = header], [Content = Text.ToBinary(body)]),
AccessToken = Json.Document(response)[access_token],
JsonResponse = Web.Contents(url, [Headers =[ #"Content-type" = "application/json", #"Accept" = "application/json", #"Authorization" = "Basic"& token], Content = Text.ToBinary(body)]),
Result = Json.Document(JsonResponse)
in
Result

I have also set the source credentials to Annonymous because if not, I cannot use Web.Contents and if i remove Web.contents and use basic credentials I get the error that data of type record cannot be converted into number. 

Does anyone has a suggeston? 🙂 I have went through a lot of community answers about such topic, but unfortunately without sucess! 

Thank you already in advance and have a nice day. 

Kind regards, 

Iheb Kharab