the app settings for blazor wasm is just the deployed wwwroot/appsettings.json file. your deployment or build system can create the correct file. Blazor WASM also support setting the env via the Blazor.Start javascript on the host page, or http header (blazor-environment) to define the environment.
to add a custom header to static web apps you configure its settings:
https://learn.microsoft.com/en-us/azure/static-web-apps/configuration#global-headers