When would a completion action on a custom redirect be triggered?

A completion action on a custom redirect will be triggered when an existing prospect clicks on the custom redirect. A custom redirect is a trackable link that can be used to measure the engagement of prospects with external content, such as a website, a PDF, or a video. A completion action is an automated task that can be performed after a prospect takes a certain action, such as clicking on a custom redirect. Completion actions for custom redirects will only apply to existing prospects, meaning prospects who have already been identified by Marketing Cloud Account Engagement through a form, a landing page, or an email4. References: Marketing Cloud Account Engagement Custom Redirects