Hello,
If you don't know the password, and didn't set up recovery method, and don't have enough history record to prove account ownership, you would just have to let that account close on its own when it hits the 2-year mark of inactivity.
Microsoft's account activity policy is explained here: https://support.microsoft.com/topic/microsoft-account-activity-policy-7c0a9fa7-0982-b7c6-fd72-df852b63699f