We have an express route in place connected with azure. Right now we are not connected or synced with azure ad. We are planning to host application into the app service (Isolated environment). We need to enable this application to authenticate with on premise AD, is this possible? if not any other suggestion would be helpful.
Your life would be easier if you could use AAD. If you don't use AAD, you're going to have to build custom authentication and authorization into your web app.
In theory, you could enable Active Directory Federation Services for Windows Active Directory, and use SAML or OpenID Connect. Unfortunately, I personally have never seen this done so proceed at your own risk.
If you are open to AAD, we make it easy to get it implemented.