Chat for Service Desk can be configured to allow customers to register in your Jira as users, directly from the chat widget, by only providing their email address. This may pose a security challenge, because malicious users can spam your Jira with multiple bogus user accounts.
This problem can be prevented by requiring registering users to solve CAPTCHA challenge. This is done by checking the CAPTCHA option in the Chat's global settings:
Chat uses Google's reCAPTCHA service to provide this functionality. Therefore, your JIRA must be able to directly perform network calls to reCAPTCHA webservice address at https://www.google.com/recaptcha/api/siteverify. Otherwise user registration will always fail.
When you enable CAPTCHA challenge, user registration dialog will contain a CAPTCHA that has to be solved, like shown in the screenshot below: