need help on HTTP Status 403 - Access is denied

I am trying to login to one of our web application. I am getting error HTTP Status 403 - Access is denied. With same user name and password, I am able to login through chrome or mozilla. But is getting error when trying to access through python. Kindly suggest

import requests
from bs4 import BeautifulSoup as bs
url="http://ift.tt/2aY12Mo"
response = requests.get(url)
print(response)
r = session.get(url)
r = session.post(url,headers=header,data={'Command': 'SIGN IN', 'username': 'myname', 'password': 'password'})
print(r.text)