When closing an opportunity as won or lost, a dialog window will pop up and looks like this:

While this dialog is set by Microsoft, there is a way to customize the pop-up dialog contents. To change the pop-up dialog window, you will need to replace it with the Opportunity Quick Create Form, which you will customize. Follow the steps below to make the customizations you prefer.

1. Go to Settings> Advanced Settings > Click the dropdown arrow next to Settings. Select Administration.

2. Select System Settings.

3. Select the Sales tab. Select Yes for the setting Customize close opportunity form. Select Ok.

4. Edit the form by navigating to Settings> Advanced Settings >Select the dropdown arrow next to Settings > Select Customizations.

5. Select Customize the System. 

6. Expand Entities. Expand Opportunity Close and select Forms. Under Active Forms, select Opportunity Close.

7. Once you have made the desired changes to the form, select Save and Publish. The changes you made to the form will apply to the Close Opportunity pop-up dialog window that you see whenever you close an opportunity going forward.