Setting up Nuki to work remotely trough Bridge

I have the Smart Lock and the Bridge.

I would like to setup Nuki to be able to open doors remotely by sending people keys they can use.

I did find some posts that seem to describe this proccess, but they seem to be outdated as the screenshots from them seem older (they are from 2018) and I am having some difficulties, so I hope somenone could help me here. I figured I would need to get the ACCESS_TOKEN, but every attempt to authenticate it has failed.

Do I need to apply for OAuth 2 API Secret?
When applying for OAuth 2 API Secret, what should I put as callback URL or the OAuth2 redirect URL? My website or something else?
And can any requests be processed until applying is succesful?
The docs say that is the URL to which users will be redirected after they successfully logged in.

If/When I succeed in getting the access token, do I have to repeat the steps when they expire since they have “expires_in”:SOME_TIME in the request response?

Do I have to create users after getting the access token so I can send emails to them? Does that have to be done for every person? And do they also have to install the Nuki App on their smartphone?