SharePoint Online - Rest api auth

Hi, 

I need to make rest requests to SharePoint from a simple command line utility (C#).

I've used CSOM/Caml with OfficeDevPnP previously but now need to perform rest calls to lists. 

I used this code to authenticate:

 AuthenticationManager authMgr = new AuthenticationManager();
 ctx = authMgr.GetSharePointOnlineAuthenticatedContextTenant(spsite, spusername, sppassword);

How do I authenticate for rest calls over HttpClient?

Tks