The Marketing Cloud Account Engagement function that should be used to track prospect engagement on a banner ad on a third-party site is custom redirect. A custom redirect is a feature that allows you to track and measure the click-through rate of any online marketing content, such as banner ads, social media posts, or email links. You can use custom redirects to create trackable URLs for your banner ads, and monitor how many prospects click on them. You can also use custom redirects to perform actions on the prospects who click on them, such as adding them to a list, assigning them to a user, or changing their field values