The Backlink Page column in LinkAssistant shows the exact URL of the backlink page on which the backlink is (or will be) placed. So, when you run the initial Search for Prospects that you don't have the backlinks from yet - the column will remain empty.
You can see a URL in the Backlink Page column if:
You can also add the Backlink Page manually to any Prospect - this can be:
In addition, you can add additional details to the Backlink: assigned Anchor Text and Linked Page (this can be useful for further verification and monitoring):