You may not cast Protect the Negotiators without a spell to target.

Once Protect the Negotiators starts to resolve, its instructions are followed in the order they're written. If it was kicked, the Soldier creature token will be on the battlefield when the amount of mana the target spell's controller needs to pay is determined.

If a card or token enters as a copy of a permanent, the new permanent isn't kicked, even if the original was.

The kicker ability doesn't let you pay a kicker cost more than once.

If a spell's kicker cost was paid, the spell is "kicked."

To determine a spell's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases (such as kicker), then apply any cost reductions. The spell's mana value remains unchanged, no matter what the total cost to cast it was.

If you copy a kicked spell on the stack, the copy is also kicked. If the copied spell is a permanent spell, the token the copy of that spell becomes when it enters is also kicked.

If you put a permanent with a kicker ability onto the battlefield without casting it, you can't kick it.