If you are looking for denied permission to limit the public access of the webapp while creating the app, you can set the following settings
If you are looking for applying the access restrictions, you may follow the following reference to help out!
Manage access restriction rules in the portal
If you are looking for restricting any access from internet, you can also setup an azure VPN to securely connect from your workspace. You may refer an article I have created on this here: Create Point-to-Site -P2S VPN Connection in Azure
Hope this helps.
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