Hi,
If you sign in to Outlook Web or Outlook Live using a non-Microsoft email address, such as Gmail, Outlook will create a temporary account that appears as a random string of letters and numbers. To avoid this, you need to sign in with an Outlook.com email address.
If your Microsoft account does not already have an Outlook.com email linked, you should add one by creating a new Outlook.com alias. Once the alias is created, use that Outlook.com email to sign in.
However, if you create a new Outlook.com account separately, it will automatically generate a separate Microsoft account and will not be linked to your existing one. Therefore, the best approach is to add a new Outlook.com alias to your current Microsoft account and use that email to access Outlook Web or Outlook Live.
If you need more help, please let me know.
Stay Safe!