Personalised message on RSVP confirmation

I would be surprised if this is possible in the confirmation box that pops up. You’d need the ability to pull in a snippet that references user name, which they haven’t ever referenced before. 

There are ways you could do this by crafting a confirmation email via an automation with Zapier or other solutions. The trigger / webhook event for when someone registers will give you their username so that you could then insert it into an automated email confirmation. If you wanted to reference other profile fields, you could add a step in the middle to look up that info from their profile.