Forgestoker and combat

[Steerpike]

So I declare attackers, opponent declares blockers, and I respond by activating {Forgestoker Dragon}'s ability.

This is the relevant rule:
506.4a: Once a creature has been declared as an attacking or blocking creature, spells or abilities that would have kept that creature from attacking or blocking don't remove the creature from combat.

So just clarifying, I NEED to activate the ability before blockers are declared for the "can't block" effect to work, right?

[Steerpike]

Ok, thank you