We are not experts in Xamarin, but we do have many clients who successfully use Xamarin with Appetize.io.
The extension we are looking for is .app. Once you have that file, you may zip it up and upload it to Appetize.
If you are trying to generate that file, we recommend you first try running your Xamarin app in the iOS Simulator on a Mac Computer. That process will generate the required .app bundle for you. Then you can locate it, zip it, and upload to Appetize. It is often found in a path like:
[Your iOS Project Folder]/bin/iPhoneSimulator/Debug/[Your App Name with extension iOS or .app]
Please check the following link for helpful tips.
Or, you can do a general Google search at https://www.google.com/search?q=xamarin+appetize.io