I have a quick question on how to get the stays when paying with an AMEX & Visa card?

Here is what I did - but did NOT get the 4 nights counted towards the MegaBonus

- checked-in on Monday at the hotel using my AMEX
- got the bill under my room when I woke up on Friday morning
- went to the lobby, got a twix bar, and asked them to add it to my room
- they added it to my hotel room (shown on the bill), and I asked them to just charge that item on my Visa card
- I ended up having the bill with room charges + snack; room charges are on my AMEX and snack on my Visa card
- but the nights did not qualify for the stay

What should I do differently to have the nights counted towards the MegaBonus, while having the room fees charged on my corporate AMEX?

Thank you!