Virgin Media doesn't tell you your Netflix details. You either use an existing Netflix account or create a new one. How that works depends on which option you have.
For bundle inclusive Netflix you'd follow the email activation link (or follow the My Virgin Media website) to either link an existing Netflix account to your Virgin Media services or to create a new username on the Netflix website and then do the same.
For the full priced 'add to bill' option, you sign up via My Virgin Media or via the Netflix app on the Virgin TV box, or you use an existing Netflix username that doesn't have an active subscription, choose the tier of service and confirm the billing to your Virgin Media bill.
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I work for Virgin Media - but all opinions posted here are my own