💡 Learn more : Azure Functions Documentation.
Generally when I'm working with Azure Functions and Visual Studio and need to add an Application Setting, then I'll head over to the Azure Portal, click on my Function, Configuration and add the Application Setting.
Over the weekend I was about to do the same and noticed that you can actually apply Application Settings within the IDE. Just go to Publish and then click on Application Setting.
You'll now see a list of Application Settings that you can add, edit or delete. Easy...