You can’t. The best you can do is pick the region / location. Read the bottom for a way to try and influence it.
A scale unit, stamp or tenant is something which I have referred to here. “How to disable TLS 1.0 on an Azure App Service Web App”
The scale unit is identified by the number highlighted in red in Figure 1 and is found by doing an NSLOOKUP on your Azure App Service name.
Figure 1, what is my Azure App Service stamp
The Address is the VIP to the stamp that you use for an A Record, I discuss that in more detail here. “How to get a static IP address for your Windows App Service Web App” do not confuse that with the outgoing IPs which I discuss here. “How to find your outgoing Azure App Service IP address”
When you create a new App Service Plan (ASP) you choose which Resource Group to place it into. All ASPs in a given Resource Group will be deployed to the same scale unit. So, my logic is, if I want to put the ASP onto a different stamp, even though I/YOU CANNOT select it, there is a probability that it is placed onto a different scale unit. You can at least give it a shot if you need that for some reason. But you cannot choose it specifically unless you create an App Service Environment (ASE).