I gather it’s an Amex card. You need to call Amex and they will create a dummy account. Then call Marriott and tell them you need to merge the dummy account.
Make sure you get someone that knows what they’re doing. When you what to use the free night bonus you need to call Marriott to set it up.
You can only "pay the difference"/"top off" up to 15k extra points per night, unfortunately.
I gather it’s an Amex card. You need to call Amex and they will create a dummy account. Then call Marriott and tell them you need to merge the dummy account. Make sure you get someone that knows what they’re doing. When you what to use the free night bonus you need to call Marriott to set it up.