How to create a custom "Document Property"?
You mean content control. Go to Word options, and in Customize Ribbon, activate the Developer tab in the right list. Then, get back to the doc and open the Developer tab.
In the Controls group, click Plain Text Content Control. This adds the text input box. If you want to give it a name like the screenshot, turn on Design Mode, select the box and then Properties.
There you can edit its title and other stuff.