Please contact Support to switch this feature on for you. If you would also like to make it required (i.e. the user MUST fill in the data field to save/update, then you can ask for this also.)
To modify the fields of a property go to your Properties tab, click on the property address and tap Edit Property.
Additional fields can be added on the bottom left of the Edit Property page.
To add a new field, click the Add new field dropdown and you'll be able to choose from all fields you've configured across your entire property database to add values for.
You can choose to create a new field, and also choose whether that field is searchable. If you make a field searchable, it will be able to search on your Properties list and Filters.
For any fields filled out on a property we'll display that information on the sidebar of your Property Details page:
You may also access data stored on your property fields via your Templates. Read about Template Merge Tags here.