Azure 403 forbidden

On the Orders API on specifically the AppUserCustomers endpoint when trying to filter by CustomerNumber I noticed that when the customerNumber has a "-" the API will respond with a 403 Forbidden. I also tried encoding the character and that did not work as well. On bringing this up to Raul Junco Villegas he pointed me in the direction of the Gateway. I tried doing the same action without going through the gateway by using the AzureWebsites url and I no longer get the 403 forbidden. After some conversation, the current suggestion is to add a rule to the gateway possibly?

What would be the best way to move forward on this one?