There was a ask from many customer to customize Opportunity Close Dialog Box as they want put some more information while closing it.
To Customize Opportunity Close Dialog Box, first we have to enable in under System Settings.
Open CRM –> Navigate to Settings –> Administration –> System Settings.
In Sales Tab, you will find the setting as Customize Close Opportunity form and by default it is set to No. Make it as Yes to customize.
Hope this helps.