The Re-Direct URL is set here as below via Dashboard > Lists > Your List > Overview > System Pages & Emails > Dropdown list top right hand corner > Subscription Confirmed > Custom Assets > Re-Direct URL
You then choose whether to send out the “Subscription Confirmed” page/message (nothing to do with Double Optin Confirmation) or Re-Direct to your Welcome page. The Re-Direct URL is entered as below. That URL CAN have variables added to it to be forwarded to the Re-Direct Page, as illustrated below. That redirect will be effective for all new subscribers.
Should you wish to redirect a subscriber who is already in the List, then you can set that as below. Those already in the LIST will ONLY be redirected if this second redirect URL is entered as below. IF you wish, that URL CAN be to the same page as the one for new subscribers.