Implementing authentication for subscribing to an iCalendar feed

We would like to write a REST endpoint which the Outlook desktop client can subscribe to and pull iCalendar data. However, because this data may contain sensitive private information, we need to implement some form of authentication.

Our REST service currently only supports password-based authentication, so we would be fine using basic authentication over https for example, but it seems that Microsoft has disabled basic authentication in newer Outlook versions.

https://video2.skills-academy.com/en-us/exchange/clients-and-mobile-in-exchange-online/deprecation-of-basic-authentication-exchange-online

I have been unable to find any information on how to go about this using a modern authentication method. However, on the following page it says this:

"By default, Outlook 2016, Outlook 2019, and Outlook for Microsoft 365 use a new modern authentication stack. Even if the Internet calendar file that you are trying to open does not require authentication, Outlook broadcasts its ability to do modern authentication on the server that contains the calendar."

https://video2.skills-academy.com/en-US/outlook/troubleshoot/calendaring/cannot-add-an-internet-calendar

Where can I find documentation on what this broadcast looks like and how the server should respond? Is there some example I can refer to that shows how this should be done?