I believe this is related to the usage of relative path in the AppDomain segment of the add-in manifest. The relative paths such as https://httpbin.org/anything is not allowed and instead you'd need to just use the domain name https://httpbin.org/. Do you need to open the new window as part of the app or could you post the form to a different frame? In other words, could you explain the desired behavior of the form and post actions?
