Form Authentication using HTML Client App to ASP.Net Web API (MVC)

Form Authentication using HTML Client App to ASP.Net Web API (MVC)

I am in a little bit situation now..
I have one web application (HTML, CSS, JS). I have one Log in form there.
This is what I need to do.
When user enters username and password and Submit Username and Password
(string user, string pass) has to be posted in api/authenticate..
api/authenticate will check the username and password either in Database /
Active Directory.. If the combination is validated the success token will
be passed back to web client (HTML app). The success token will be used as
authentication flag throughout the application.
I have came across many solutions but couldn't determine which is the best
approach to achieve this.
Any solution would be helpful :)
Regards..