How do I connect the Azure App service to the Local OnPremise Corporate network using ExpressRoute?
You need to use the App Service VNET integration feature to access an Azure VNET resources from a web app. Then connect that VNET to your on-premises network with ExpressRoute and set up the correct traffic routing.
https://docs.microsoft.com/en-us/azure/app-service/web-sites-integrate-with-vnet#access-on-premises-resources