Where do I find ServerVariables("REMOTE_USER")?

I used Request.ServerVariables("REMOTE_USER") in classic ASP to get the id of the logged in user.  I can't figure out where to get the information in ASP.NET Core.  I've reviewed the HttpRequest class documentation at https://docs.microsoft.com/en-us/aspnet/core/api/microsoft.aspnetcore.http.httprequest#Microsoft_AspNetCore_Http_HttpRequest but there isn't a ServerVariables property.  

If anyone can point me to how to get this information I'd really appreciate it.