How to play the Harbinger card in Dominion
Short answer: yes, you can draw it immediately with, e.g., another action card.

Long answer: the golden rule in Dominion is to follow a card's text entirely, top to bottom, before moving on to your next play. In this case, you:


Play the Harbinger.
Draw a card from your deck, shuffling it if empty.
May optionally put a card from your discard pile on top of your deck, if there are any cards in your discard.
Play subsequently another action card, such as a Village, to immediately draw that card.