Issue
How do the links (both Marketo and Non-Marketo) populate in the dropdown of Visited Web Page trigger or filter?
Solution
The drop-down shows a limited number of options. These links are fetched from any of your leads' activity logs. The values in the dropdown of the "Visited Web Page" will show if there is any lead who visited a webpage and this webpage or link is stored in the activity log of the email (may be in past).
If the expected webpage doesn't show up in the dropdown list, first try typing it in and letting the autocomplete offer the URL. If you type it in and it still doesn't show as an option, then none of the leads have visited that page at least once. You can create a test lead for yourself and visit the desired webpage to get this added in the dropdown.