I created an app on https://apps.dev.microsoft.com
with the following Application Permissions:
Calendars.Read (Admin Only) Calendars.ReadWrite (Admin Only) User.Read.All (Admin Only)
The following is the only flow that has worked for me to be able to subscribe to notifications of another user, on another tenant, as described here
Admin Consent
Admin consent was then successfully granted via this URL
https://login.microsoftonline.com/common/adminconsent?client_id=bbb35336-faee-4c10-84b4-34136634db41&state=1234&redirect_uri=https%3A%2F%2Fdashmeetings.com%2Fmicrosoft%2Foauth
Get access token
An access token was then obtained from
POST https://login.microsoftonline.com/{tenant}/oauth2/v2.0/token
with headers
Content-Type=application/x-www-form-urlencoded
with {tenant} the value of tenant returned in the callback url,
and body with key-value pairs
grant_type=client_credentials
client_id=bbb35336-faee-4c10-84b4-34136634db41
client_secret=xxx
scope=https://graph.microsoft.com/.default
This returns an access token, but not a refresh token.
I think this might be because offline_access isn't requested.
How can I get a refresh token?
You're partially correct, you will only receive a refresh_token if you request the offline_access scope and you are using the authorization_code grant flow.
Refresh tokens are not available when using the implicit grant and are unnecessary when using the client_credentials grant. When using client_credentials there isn't a user authenticated and therefore there isn't a need to "refresh" a token since you can simply request a new token when needed.