File >> Info >> Properties >> Advanced Properties
Go to the "Custom" tab and enter your new property. Don't forget to hit "Add" at the end!
To have this field appear, you would use CTRL+F9 to create Curly Brackets and insert DOCPROPERTY Counter inside:
Then you would press F9 to change to the counter 0
- If you go back and edit the field, you can also right click and hit "Toggle Field Code" and "Update"