Make it a separate field (looks like you have this already) and make the field or layout text a button that uses Open URL to open the page. You can set up a calculation inside that Open URL step that correctly accesses the URL for the current record.
I'm not sure how to do that. Is there a step-by-step reference I can access?
You can look up Open URL in FileMaker Help.
Do you know how to create a script?
Do you have the URL stored in a field in the layout's table?
If you want to see an example of how you might do this, download the Known Bugs List database (click the blue down load button), click on the Bug Reports button and take a look at how the Open URL buttons function on this layout. (Enter layout mode and double click the buttons...) These are set up in much the same way except that they are regular layout buttons instead of fields setup to be a button.