How to create User Authentication in web API

These Tutorials are very helpful for me to understand Web Api more:

http://www.c-sharpcorner.com/article/xamarin-android-create-login-with-web-api-using-azure-sql-server-part-one/

http://www.c-sharpcorner.com/article/xamarin-android-create-login-with-web-api-using-azure-sql-server-part-two/

But what if i already having an API? (Self host/ WPF form):

I already have this API, what I only want is to create a validation function using this self hosted API, But in the tutorial, all of the function of login validation is in the Azure form, can someone give a link that is close to this? or translate the code?

Thanks in advance