Step 1: Click on "Settings" (gear icon) in the top right corner of any page.

Step 2: Click the Custom Fields header.

Step 3: click the "add" button in the Unit Custom Field card.

Step 4: Type in the custom field name you are adding & click the blue Save button.

Note: Unit custom fields are searchable on the Units page and can be exported to CSV or PDF by going to Reports --> Custom Report.