Just make sure your expression returns a value that the action accepts for this parameter (remember that when hovering an action cell in the editor, you can see the raw content of each attribute : this is the kind of values your expression has to return for it to work properly).
Expressions still follow the syntax explained sooner in this topic, which enables calls to variables, usage of functions, etc.
And by the way, checking the "Expression ?" box replaces the 'xpr=' prefix you had to use on some actions before (Define a variable and Test a variable's value), so you shouldn't write it anymore.
