I struggle in call the REST API at Dotmailer.
he provide a document for call the OAuth and get the token. but it's always throws the same error.
Input is
Post Method
URI: https://r1-app.dotmailer.com/OAuth2/Tokens.ashx
JSON input is 
{
    "client_id" : "apiuser-XXXXXXXXXXX@apiconnector.com",
    "redirect_uri" : null,
    "client_secret" : "XXXXXXXXXXXX",
    "grant_type": "authorization_code",
    "code":"MyServerCode",
    "test_mode":true
}
Out put is 
{
  "error": "invalid_request",
  "error_description": "There was a problem with the 'grant_type' parameter: The parameter is required."
}
Anyone know the error.
Thanks in advance...