@Rob Nicholls
Thank you for your post!
Azure Defender doesn't support enabling this feature for specific App Services, as of right now. You can only enable Azure Defender on your subscription or you can optionally enable this only for the App Service plan, as described in Pricing of Azure Security Center.
If you'd like to create a feature request you can do so using our User Voice forum.
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.