n the previous article Azure App Service: Create Web App from Visual Studio we have learned how to create a new Web App. In this article, we will learn how to link an existing web application to an App Service Web App.
Please go through the how-to-create-a-web-app-in-azure  link and create a new App Service Web App.
Go to Settings Blade of the App Service Web App. Click on “Get Publish Profile” as shown below.

Below is the screenshot of the file that I just downloaded.
Open the Web Application that you would like to link the Web App in Visual Studio. Right click on the project and click on “Publish”.
In the “Publish Web” window, click on “Import” button.
A popup will be as shown below.
Clicking on the “Browse” button in the above screen, a windows will be opened as shown below. Please select the publish profile that we just downloaded.
Once you select the file, the path of the file will be displayed as shown below.
Once you click on “OK” button in the previous step, the Publish window will configure the App Service and the Visual Studio Project.

That’s it. It’s how it is to link an existing web application to an existing or a new App Service web app on Azure. Once you click “Publish” the codes get deployed to the App Service for which you downloaded the Publish profile.