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It should be possible to modify the Close opportunity form. The same way it is possible in April19 release to modify the close case form : https://docs.microsoft.com/en-us/business-applications-release-notes/April19/service/dynamics365-customer-service/case-management-case-resolution-extensibility
Category: Lead Management
STATUS DETAILS
Completed
Ideas Administrator

Thank you for your feedback!

 It is now possible to customize the close opportunity dialog. Check out Customize the Opportunity Close form.

Sincerely,

Bharath Varadarajan

Product Manager, Dynamics 365 Sales