Keldon Flamesage

To enlist a creature, that creature must be untapped, it must not be attacking (even if it has vigilance), and it must have haste or have been under that attacking player's control since the beginning of their current turn.

If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as a kicker cost. If the card has any mandatory additional costs, you must pay those to cast the spell.

When a player taps a creature for an attacking creature's enlist ability, that attacking creature gets +X/+0 until end of turn, where X is the tapped creature's power. This is a triggered ability that goes on the stack immediately after attackers have been declared in the declare attackers step.