A wizard can both predefine variables via the Set tag or by using the result tag in panels that accept input (choicePanel, filePanel, stringPanel).
Once defined, variables are referred to with a '$' prefix. Thus, once the variable ExportFolder is defined it is called as $ExportFolder.
Wizard variables can be concatenated with text strings:
…but note that strings shouldn't be quoted (otherwise) the quotes appear in the resulting output.