You can create a session variable and set it to the value you want the hidden form element to be and use the like entry server validation to compare the hidden element against the session variable:
Go to the bindings panel. Click the plus button and select "Session Variable". Name the session variable "Subject".
On you page add the following code at line 1 to set the session variable:
if (!session_id()) session_start();
$_SESSION['Subject'] = "Here's your requested link";
In the server validation wizard, select the hidden form element, and select the Like Entry validation type, when you click the plus button to add it, select the Subject session variable.