It seems OAuth2 hasn’t implemented yet. At this time, to get auth value, username and password are required:

# Python
url = 'https://<instance_name>/user/login'
data = {'username_or_email': username_or_email,
        'password': password}
response = requests.post(url, json=data)
json = response.json()
auth = json['jwt']

Since the endpoint recieves JSON then returns JSON, you may need to send following HTTP headers explicitly:

Content-Type: application/json
Accept: application/json

There doesn’t seem to be a way to access eg the communities tab via activitypub? Any Help?

If you’re writing client-to-server part (not server-to-server part), try curl 'https://<lemmy-instance>/api/v3/community/list?sort=Hot', and an API client libraries written in Python..