3rd part email address settings to login on Outlook or similar.

https://www.virginmedia.com/help/broadband/manage-email-account

The settings can be found in the help link above.

For access via an app such as MS Outlook the username remains your full Ntlworld email address not the third party email username.

You need to generate an app password to use with email clients.

VM have recently changed the log in requirements for accessing VM Mail via third party apps.

If you change the password for the account in My Virgin Media > Account settings > Account details you will not be able to use the new password to access the account via an email client. Email client access now requires an app specific password which can be generated in the MyVM > Account Settings > Account details > Mailbox App password management

The third party email username is only needed for accessing your My Virgin Media account and the VM webmail page both of which require the account password not the app password.