Please note the highlighted area under System info where it says Machine name and Instance id I choose the first instance in the dropdown menu, and clicked Environment to see all information about that particular instance.
Important: these 4 characters come from the ARR Affinity cookie, so you can use the first characters to identify the instance and choose properly. As you can see it shows all instances of your App Service, in my case 2. Advanced Tools will open in a new window, and it looks like the image below, since you have scaled out your environment, you will see Instance: 4 characters and an down arrow, click on it.
