[Python] How to use a web proxy with HttpNtlmAuth - auth_header_value is negotiate

I have the below. If I take out , proxies=proxies and try and connect to the Intranet, it works, I get a response back.

However if I try an external site with the above put in (as per my example below), I get a 407 error

import requests
from requests_ntlm import HttpNtlmAuth
proxies = {'http': 'http://myproxy.local:9090'}
ntlm_auth = HttpNtlmAuth('DomainName\\MyUsername','MyPassword')
res = requests.get("https://bbc.co.uk",auth=ntlm_auth, proxies=proxies)
print(res.content)


Am I doing something obviously incorrect? When I go to IE and look at the proxy information there, this is exactly what I am using

UPDATE

import requests
from requests_ntlm import HttpNtlmAuth
ntlm_auth = HttpNtlmAuth("DomainName\\MyUsername","MyPassword")
proxies = {'http': 'http://myproxy.local:9090'}

s = requests.Session()
s.proxies = proxies
s.auth = ntlm_auth

res = s.get("http://bbc.co.uk")
print(res.content)


I get the following:



When outputting the value of auth_header_value I get negotiate

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