I personally would not waste it on the undercarriage. It probably won’t make it in the vents like it will if you inject a fragrance with soap/wax/rinse cycles. It would only be noticeable to people outside the car if you inject scent in the undercarriage spray.
That add injector is probably intended for a rust inhibitor type chemical. Not saying that will do a whole lot, but most likely what some operators would want to use it for.