Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
You can use this instruction to look up all usernames you might have, and then follow the AI instruction to see if it might be in one of those accounts: https://support.microsoft.com/en-us/accounts-billing/manage/forgotten-your-microsoft-account-username
If you still cannot find it, any chance your subscription was purchased via Apple App store, or Google Play? If so, they will not be listed in your Microsoft account, but in your Apple or Google account instead.