Filemaker Phone Number Auto Format (Mask)

[@jskooler]

Hello,
I am using FMP Adv 11.0v2. on Apple OS 10.9.5
When trying to add the line: Set Field By Name [$$ActiveField; $$FieldContents & "-"]
It won't let me specify the target field as above [$$ActiveField; $$FieldContents & "-"] because it expects an operator in place of where the semi-colon is. Any idea on what might be done to change this? I've been looking at the syntax of Set Field By Name but haven't come up with anything clever yet.
Thanks,
Dan

[@courtlandcarson6610]

Hi there. How could I modify this script to format the phone number as (###) ###-#### ?