Google reCAPTCHA protects you against spam and other types of automated abuse. It is used to stop spam on a website and verify whether a user is a human.
This article provides an example of implementing invisible reCAPTCHA on an ASP.NET Login page.
There are three possible types of invoking invisible reCAPTCHA: automatically bind the challenge to the Login button, programmatically bind the challenge to the Login button, and programmatically invoke the challenge.
In this document, the challenge is invoked programmatically after pressing the Login ASP.NET button on the form.