@BRUNO BANDEIRA 

Could you please set your request body and header as follow:

var requestBody = {
    "grant_type": grantType,
    "username": username,
    "password" : password,
    "client_id" : clientId,
    "auth_mode" : authMode
}
req.setRequestHeader('Content-Type','application/json');
request.setRequestBody(JSON.stringify(requestBody));