Two methods of getting FTP details for an Azure App Service are below.
Download FTP details via ‘Get publish profile’
Navigate to your App Service and click ‘Get publish profile’ along the top navigation bar. This will download a .PublishSettings file which will contain all your FTP details including host, username and password.
View FTP details via Deployment Center
You can also view your details via Deployment Center which is under the Deployment link on the LHS. Select FTP and click Dashboard at the bottom and Azure will show a popup with your FTP details.