My ground instructor certificate number is not the same as my pilot certificate number. None of my five FAA certificates have the same number.
Put "pending," but also put your pilot certificate number. The certificate number is as much for your identification as it is validation that you're really a certificated instructor. Tracability. If an inspector ever looked at that endorsement, he couldn't look you up in the system to verify you using the word "pending," but when he pulls your information using your pilot certificate number, he will have everything he needs right in front of him.
Don't use your pilot certificate number instead of your instructor number, but rather put "pending," and below that for good measure note your pilot certificate number and level of certification (eg, commercial, or ATP). That is not a requirement, but a very good idea.