You can create the pop-up to have links in that format when you are making the map. In the browser while making the map, configure the popup for the layer where you want this behavior so that it includes the link.
In the following example, we'll use ID_NUM as the field on your feature that includes the value you want used in the link, and we'll create a popup that reads "See more details about feature X online." where "online" is a link to the page including the ID:
In the pop-up properties, set the Display drop-down list to "A custom attribute display"
Click the "Configure" button below the Display setting. This opens the Custom Attribute Display dialog.
Type "See more details about feature "
Click the + at the end of the toolbar for the dialog and select the field with the ID (here I'll use ID_NUM). This inserts {ID_NUM} at the end, leaving the sentence: "See more details about feature {ID_NUM}"
Type " online.", making the sentence "See more details about feature {ID_NUM} online."
Select "online" in your sentence, and click the Create Link tool on the toolbar.