The top card of your library is still in your library and not in your hand. You can't discard cards from the top of your library or activate abilities of cards on top of your library that normally work in your hand.

You must pay all costs and follow all timing rules for cards played from the top of your library this way. For example, you may play a snow land this way only while the stack is empty during one of your own main phases, and only if you haven't played a land yet this turn.