Hi,@Graham, Laurie Welcome to the Microsoft Q&A platform, thanks for posting the question
I Understand you are having trouble finding "MySQL in App" inside the settings section of your Azure portal resource.
"MySQL in App" is a feature of Azure App Service that allows you to run MySQL databases in the same environment as your web app. I think you are using a Linux-based Web App, MySQL in-app is not available for Linux App Service plans.
You could either create a Windows-based Web App or use a Docker container to host both, your application and a MySQL database on Web Apps then you can find MySQL in App as shown below.
I
I hope this information helps
Regards
Geetha