For Microsoft office 2016 & 2019, a Microsoft account is required to be tied to a Microsoft Office license key. Hence, should you lose/forget the account that is tied to the license key, you may follow the below steps accordingly to transfer the license key to a new account
If you forget the account, kindly contact Account/ Billing team via Online support at https://support.microsoft.com/en-us/contactus/
Please type Talk to an Agent. As the conversation goes on, you will see the following option comes up that ask you to choose the product and issue in the chat, shown as following pictures:
You can choose Outlook as the product and Technical support as the issue like below, then click Chat, you will be able to talk to a real agent.
Type in “Talk to an agent” >> Choose the product for “Office for Home” >> Choose topic “Account /Billing” >> Click on “Chat” talking to Online support (Real agent)
Provide them your Proof of Purchase and Product key to check the Account issue.