When you create a filter on a custom resource, in order to create a parameter, you have to start the process of adding a rule, check the Switch to Parameters checkbox, then select an existing parameter or define a new parameter by typing a name and clicking the plus sign. (Configuring filter definition)
However, if you want to use a parameter in an expression entered in Advanced Mode, there is no place to create the parameter in the Advanced Mode dialog. You have to first create the filter condition in Simple Mode, then switch to Advanced Mode.
In addition, there is also no way to create parameters first, then reference them in conditions.
It would be more flexible and convenient if it were possible to create parameters without first creating a condition in Simple Mode.
By the way, I couldn't find this in the documentation, but to reference the parameter in the expression, use this: $(@Abc_parameter), where "Abc" is the parameter name.