Explanation/Reference:
The Set-WebApplicationProxyApplication cmdlet modifies settings of a web application published through Web Application Proxy. Specify the web application to modify by using its ID. Note that the method of preauthentication cannot be changed. The cmdlet ensures that no other applications are already configured to use any specified ExternalURL or BackendServerURL.
References: https://technet.microsoft.com/itpro/powershell/windows/wap/set- webapplicationproxyapplication Explanation 2:
Configure internal and external Fully Qualified Domain Names (FQDNs) , there are two FQDN addresses that are configured with an application. The External URL is the FQDN that external users request access to when attempting to access an application. The backend server URL is the FQDN of the internal resource where the application is available. In most scenarios, these URLs should be the same. If the FQDNs are different for external and internal requests, then URL translation must also be configured to ensure requests are redirected correctly. To enable URL translation, use the Set- WebApplicationProxyApplication cmdlet. Set-WebApplicationProxyApplication -ID AppID - DisableTranslateUrlInRequestHeaders:$False