You choose whether or not to cast the target instant or sorcery card as the reflexive triggered ability resolves. If you do, you do so as part of the resolution of the triggered ability. You can't wait to cast it later in the turn. Timing restrictions based on the card's type are ignored.

Halo Forager's enters-the-battlefield ability triggers and goes on the stack without a target. If you pay , the second "reflexive" triggered ability will trigger. You'll choose the target instant or sorcery card for that second ability at that time, based on the value you chose for X. X can be 0.

If the card has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a card "without paying its mana cost," you can't pay any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, those must be paid to cast the card.