Hey there! Nice to meet you!
You can try using the username lookup from this link: https://support.microsoft.com/account-billing/forgotten-your-microsoft-account-username-b2049472-3b8f-27d3-61c6-67a668453f4c
However, the system can only show you a hint of the username once you have provided the verification codes through the account's security verification methods.
If it doesn't work, kindly try finding your Microsoft account username in other products or services associated with that account through this link.
You mentioned that you have a subscription on that account. Please be advised that unless the account owner is able to access the account, subscription cancellation is not possible, and Microsoft personnel cannot retrieve this information.
If you are no longer able to recover your account and you need to stop the recurring charges, you will need to reach out directly to your bank and request to freeze all charges from Microsoft, cancelling the subscription on its renewal in the process.
You may also need to request a new payment method.
Thank you for your understanding.
Best regards,
Darhyl