ArcGIS Experience Builder: The URL does not reference an AppBuilder Extension

Error Message

When adding a custom widget in ArcGIS Enterprise's Experience Builder, entering the manifest URL returns the following error message:

Error:   
The URL does not reference an AppBuilder Extension.

Cause

• The web server hosting the manifest URL is not included in the address list of allowed proxy hosts.
• The web server does not have permission to access the custom widget folder location.
• An incorrect manifest URL is specified when adding the custom widget.

Solution or Workaround

Note: 
The following workarounds may require the assistance of the organization's IT team.

Add the web server to the address list of allowed proxy hosts in the Portal Administrator Directory

In the Portal Administrator Directory, specify the web server in the list of approved addresses. Refer to Portal for ArcGIS: Restrict the portal's proxy capability for further instructions.

Note: 
If the web server is controlled by a third party, take into account the security implications of adding the web server to the address list of allowed proxy hosts.

Modify the permission for the custom widget file

Set the permission for the folder location where the custom widget file is stored in the intended web server. To set the permissions, refer to the Solution section in Error: The ArcGIS Server is not able to connect to the specified directory.

Specify the correct manifest URL when adding a custom widget

When specifying the manifest URL, the URL format is as follows:

https://<server.domain.com>/<widgetPath>/manifest.json