How to refresh API access token dynamically

Hi ,

How to generate API access token dynamically using refresh token or basic auth (username and passowrd) 

I tried to generate access token dynamically but failed with the below.

let

url ="https://testapi.apples.com/api/v1/auth",

headers = [#"Content-Type" = "application/json"],

postData = Json.FromValue([username="username",

password= "password"]),

response = Web.Contents(

url,

[

Headers = headers,

Content = postData

]

),

access_token = Json.Document(response)

in

access_token

Error:

DataSource.Error: Web.Contents failed to get contents from 'https://testapi.apples.com/api/v1/auth' (400): Bad Request
Details:
DataSourceKind=Web
DataSourcePath=https://testapi.apples.com/api/v1/auth
Url=https://testapi.apples.com/api/v1/auth

Any suggestions would be much appriciated.

Many thanks