Hello @Ramprasad V
Yes you can create a dashboard that can fetch data from other subscriptions (and tenants!). The authentication here is based on your individual permissions. There are various ways to create dashboard, but either pin the relevant tile you want to add or create on from the subscription you want to display. https://learn.microsoft.com/en-us/azure/azure-portal/azure-portal-dashboards
To answer your second question. Yes it can make REST API calls, but only to Azure ARM, so calling the App Service is possible, providing it is data accessible through Azure Resource Manager.
kind regards
Alistair Ross