When you pick one of the possible landing pages when sending a message, the user is taken to the page you choose, no matter what.
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It doesn't matter if that person is already an active user (already went through the registration form) or not.

When the Campaign is sent, the application creates a custom link on a per-recipient basis. 

If the user is new, then the link is a unique tokenised link that can be used only one time by the user (and that actually has an expiry date), to take ownership of the user account pre-created when you created the user in the first place, by letting the user confirm they are who you thought they were, and choosing a password.
Once this step completed, the user is then indeed automatically redirected to the page you intended to link to when you chose a "Message Link" (a particular presentation for example).

If the user is not new (she was already contacted and followed the above steps), then the link points straight to the page you selected.

Either way, the user is guided through necessary steps in a secure way, always ending at the page you intended them to land on.