Authentication with Active Directory. Angular 2, Spring - theoretical

I am doing some research about Active Directory authentication for an internal application.
The application is split into two parts - the front-end in Angular 2 and the back-end in Spring. I would like to add Windows Authentication to it to avoid providing credentials when you are already logged in, but I cannot fully understand how the server knows the user's credentials, which can match the data stored in Active Directory. > For example, let's say I'm logged into Windows, my data is sent to Active Directory.
I am now heading to a site that I would like to auto-authenticate with an HTTP request sent to the rest api.
Rest server calls some logic related to authentication in Active Directory and after successful api sends token back to Angular. But what I am missing is how the server knows which credentials are being compared to see if the user exists in AD.
Should I somehow use Angular to access the user credentials stored in windows through the browser? Or am I missing something important here.