You can create an Edit Input rule and specify it on the Advanced tab of the Property rule. In your section, you will need to add an On Change to post the value so it will show up right away; otherwise, it will not appear until the submit.
There is an OOTB appendDash Edit Input rule. It would put the dash at the end, not where you want it, but you could check it out for your initial test to see if this would work for you before you create your own.
While creating a property, you can go to the advanced tab and check the various fields to provide different validation rules. The edit input rule basically involves writing a piece of Java code, and does not require Pega knowledge.
To know how to create them in detail, go through the System Architect Essentials course on Pega Academy.