Don't do it.
Dealer-applied undercoating is not necessary on modern vehicles, "modern" meaning anything after 1980 or so. Also, an imperfect job - more likely than not due to the techniques used - can result in far worse long-term damage due to trapping road salt in voids.
Dealer "undercoating", like "paint protection" and "Scotchgard", are what are called "packs" - services of trumped-up or even questionable value that costs the dealership very little. IOW, it's just another opportunity for them to get into your wallet.