Hello, thank you for reaching out to Microsoft Q&A.
For security reasons, you will only see a partial email address when using your alternate security contact methods to find your username.
In this situation, please first check whether you have sent any emails or messages from this account to your friends, family, or neighbors. If you have, you can ask them to share the full email address shown as the sender.
If that’s not possible, open Microsoft Edge or your Google Password Manager and look for any saved sign-in details for Microsoft services (such as Outlook, Xbox, or other Microsoft apps). Check whether the credentials for your account are stored there, including the full email address associated with your live.com (or outlook.com) account.
For reference, see:
Check products or services where you used your Microsoft account