What have helped us in past is to check Blocked URL list under Security & SD-WAN settings or enter the URL under Check Content and threat categories as MX consider that link as "Finance" - If you have Finance category blocked it won't work.
Additionally, when you type in a URL under Allowed URL list it needs to be the domain only meaning in your case just type in walmartrewardsmc.ca. ** It might take a min or two but should work**
