Add-In for Outlook problem

Question

Brand new migrated Exchange Server 2016 CU19 everything works fine except for this third party add-in from our case management system software developers. I can install the add-in and login fine and return data from them displaying in outlook. the add-in is supposed to allow posting the email into their system, however when trying to post back it does not work. One of the developers told me that it is my Exchange server setup and that the error he is seeing is HTTP405: BAD METHOD - The HTTP verb used is not supported. (Fetch)OPTIONS - Access to fetch at 'https://xxx.xxx.com/api/v2.0/me/messages/AQMkADJkNGVkODliLTM4YTctNGE2My1iODJjLTY0NmRhYzg0ZTM3ZQBGAAAD-XOWCdKfqU6JePA-KI5lWwcAaCsjgqm48Uiqkxq0UYyiiQAAAgEMAAAAaCsjgqm48Uiqkxq0UYyiiQAAAgV1AAAA/$value' from origin 'https://outlook.filevine.com' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled. Anyone able to shed some light on this for me same behavior in OWA and Outlook? I have other add-in installed and they do not seem to have any problem at all posting back(may be a different method though)