TKD wrote:

You need to go to your *nominated* station or it gets refunded.

Also in that case auto top up would not be set up on your card.

Yes, the Oystercard website makes that bit clear enough.

Having tried this yesterday, I can answer my original question - which
was "will I be charged for a journey when swiping my card to receive my
auto top-up instruction?" - the answer is "yes". The barriers open
immediately upon loading the top-up.

If you just want to swipe the card to load the auto top-up instruction,
but you don't want to make a tube journey, you'll then need to go to a
ticket window and have the new journey cancelled.