Web Assist Markup

When I use ADDT extensions, I have a markup language to use in their GUI/Code that makes it easy to express complex values. For example, if I have a Session value for the first and one for the last name of a user, I can enter in the GUI:

{SESSION.fname} {SESSION.lname}

and the code comes out:

"{SESSION.fname} {SESSION.lname}"

which gets translated to:

$_SESSION['fname']." ".$_SESSION['lname']

Whenever I try to add something more complex than a simple value, I seem to have to hand code it and if I reopen the GUI I get the message that a server side value is configured incorrectly.

Is there a proper way to enter something like, "https://".$_SERVER['SERVER_NAME']."/destination.php" into web assist GUI's that the extensions will consistently accept?

Here is a screenshot example of where I would like to be able to add a value like that. What syntax works.

complex_values.jpg