How to assign a string var in agilent advanced design system

Hi everyone. I have a simple question about how to assign a string variable in agilent advanced design system (ADS).

For example, I want to assign the "port" component's "layer" attribute as an string variable "X", and I have already define "X" in the variable parameters. In this case, how can I assign the "X" to the "layer" attribute? It seems direct put that in the "port" component field, like other numerical variable, can not work.

Thanks in advance.