I don't know how stubhub's api works, but generally it should look like this:
s = requests.Session()
data = {"login":"my_login", "password":"my_password"}
url = "http://example.net/login"
r = s.post(url, data=data)
Now your session contains cookies provided by login form. To access cookies of this session simply use
s.cookies
Any further actions like another requests will have this cookie