It would be helpful to know which application you're referring to.
Here are the steps for configuring captcha in Confluence and JIRA, respectively:
https://confluence.atlassian.com/display/DOC/Configuring+Captcha+for+Failed+Logins
https://confluence.atlassian.com/display/JIRA/Enabling+Public+Signup+and+CAPTCHA#EnablingPublicSignupandCAPTCHA-EnablingCAPTCHA
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