For Platinum challenge, the qualify stay requires a "paid" stay. so does an OTA booking qualify?
Great question, JC Lam! Stays booked through online travel agents, such as Expedia, are not eligible for Marriott Rewards credit, including credit towards promotions or challenges.
No, but if you find a cheaper rate, use Marriott's 'Look No Further' guarantee so your stay can count for a challenge (and in general), info here: Online Hotel Booking and Reservations | Book Right on Marriott.com
The claim form is here: https://www.marriott.com/look/claimForm.mi
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