import requests
from bs4 import BeautifulSoup


with requests.Session() as s:
    headers = {
        'user-agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.99 Safari/537.36'
    }

    login_data = {
        "lt": "LT - 354243 - EEoajcVf5ibeUdF6M91a2U3Ss5NsWd",
        "_eventId": "submit",
        "execution": "e2s1",
        "username": "user",
        "password": 'test'
    }

    url = 'myurl'
    s.get(url)
    r = s.get(url, headers=headers)
    soup = BeautifulSoup(r.content, 'html5lib')
    login_data['_atriumconnexion_WAR_atriumportlet_formDate'] = soup.find('input', attrs={'name': '_atriumconnexion_WAR_atriumportlet_formDate'})['value']
    r = s.post(url, data=login_data, headers=headers)
    print(r.content)