Hi DatLuu1372,
Yes, in Office 365, you can directly sign in with your Office 365 account to activate the Office applications. During the process, you don’t need to enter a product key. And actually, as part of the activation process when signing in with your account, Office 365 ProPlus communicates with the Office Licensing Service and the Activation and Validation Service to obtain and activate a product key.
For your reference: Overview of licensing and activation in Office 365 ProPlus
Regarding “So is there any situation that requires product key when using Office Pro Plus in O365?”, the answer is no if you are using Office 365 cloud service. Users can just sign in with your Office 365 account to activate the Office applications directly.
Regards,
Charles