Weapon property
 

I am looking to add an item property and not sure where to start. It is similar to masterwork but can add to damage as well as to hit. I can't seem to find where masterwork or the +1 is in the editor to copy how those work.

You can't copy the mechanism for +1, but you can copy the scripts from the adjustments. There is one each for weapon damage and weapon attack bonus.

Alternatively, you can just create a masterwork weapon then use the weapon damage adjustment to add to the damage without creating a new item property.