How to get Window's logged in User Name and Password using ASP.NET WEB API 2

I am working on a Intranet application where i am calling a SAP service from a ASP.NET WEB API, Currently it is working fine with a HttpBasicAuthentication with a predefined user name and password. But now the requirement is to get the logged in windows user's user name and password through ASP.NET WEB API and pass it to the SAP service for authentication? Can anyone help me on this?