Halo Forager

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.

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

Halo Forager's enters-the-battlefield ability triggers and goes on the stack without a target. If you pay X generic mana , 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.