You can define a parameter in a number of different ways:
The Add Parameter dialog makes it easy to add a queryparm or setparm command to your query. When you add one of these commands, it will be placed at the top of your SQL at the end of the existing query queryparm / setparm commands.
You open this with (from the SQL window) Parameters > View/Edit Parameter
By default, parameters are indicated with $. On the Parameters Dialog, you can specify the character to be used.
You can also specify the character with the following command. This command might, for example, be included in a script:
The SQL window has a tab which allows you to view and alter parameter values.