We use Google reCAPTCHA v2 Tickbox.
There are 4 steps to get it done.
1. Sign in to your Google account and visit https://www.google.com/recaptcha/admin/create
2. Fill in the form as follows:
3. Click Submit, you will get your SITE KEY and SECRET KEY
4. Copy the two keys respectively, Sign in to the script as the super admin, go to Global Setting -> Auth Integration, tick Enabled and paste the two keys.
Click Save Changes.