The Nextdoor API is currently in closed BETA - learn more about requesting access here.
We have the following region servers:
Canada Server: https://ca.nextdoor.com/
US Server: https://nextdoor.com/
Australia Server: https://au.nextdoor.com/
Europe Server: https://nextdoor.co.uk/
Note that auth.nextdoor.com is global.
The Nextdoor Share Plugin is an open offering and you are free to integrate it without Nextdoor approval.