The Custom Website section uses the account's Pixel, so it's only active if a Pixel is detected. Click Enable Site Audiences and select the options you want: All visitors (with an optional field to limit it to a specific URL, such as the thank-you page), and Time on site 5%, 10%, and 25%, which create audiences of your most engaged visitors (those who spent the most time on the site). Each option is combined with the retention periods you selected. Below these options, you'll also see the Pixel Events list, which lets you create audiences from any event your pixel fires.