Unable to connect to Power bi using restapi

I am trying to send POST request to

https://login.windows.net/common/oauth2/token to get authorization code with parameters inside the body:

{
"grant_type":"password",
"client_id":"xxxxxxxxxxxxxxxxxx",
"resource":"https://analysis.windows.net/powerbi/api",
"username":"xxxxx.onmicrosoft.com",
"password":"xxxxxxx",
"client_secret":"xxxxxxxxxxxxxx"
}

I am getting error:

> { "error": "invalid_request", "error_description": "AADSTS90014: The
> request body must contain the following parameter: 'grant_type'. Trace
> ID: 6725893b-5f12-4f6b-9334-ea6c86f10400 Correlation ID:
> 27f51716-70fb-45f0-85d5-4ccfb32390c2 Timestamp: 2018-10-16 12:21:04Z",
> "error_codes": [ 90014 ], "timestamp": "2018-10-16 12:21:04Z",
> "trace_id": "6725893b-5f12-4f6b-9334-ea6c86f10400", "correlation_id":
> "27f51716-70fb-45f0-85d5-4ccfb32390c2" }

how can i solve this issue?