I am trying to perform a basic step (this works when i write an apex class and then try to get the access token, but does not when i do the same from google chrome extensions)

Please see the details:
I am hitting the URL :


And i am giving these as headers:
grant_type: password
client_id: 3MVG9Y6d_Btp4xp6.aP2dpK_QvKPMy.wCLzvqeElabZryrMYgZfEyCCMVUg5fm.6SqUW5525mr3xgXNStu2IM
client_secret: 7269476387055204688
username: --------@gmail.com
password: --------
Accept: application/json

Still getting the error:
{
  "error": "unsupported_grant_type",
  "error_description": "grant type not supported"
}

I tried adding the Content-Type parameter as well, but does not work.
Kindly help!

Regards
Rajat