Welcome to the site. What you need to do is set the appropriate "gameProperty" to "true".
<property name="Shore Bombard Per Ground Unit Restricted" value="true" editable="false">
"Pact of Steel 2" explains how to mod the xml. DL in "Download Maps" when you start up triplea. Then go to your triplea folder and open "downLoaded Maps". Open Pact of Steel and keep opening until you hit "games". Open that and POS2 xml will be there. Make your changes and restart triplea and you should be good to go.
I'd recommend reading the xml multiple times and always refer to it to really understand how it all works but for what you want to do just set the gameProperty to true.
To clarify you would make the changes to the map that you're modding obviously. I worded that poorly