Manifest - Key

This value can be used to control the unique ID of an extension, app, or theme when it is loaded during development.

To get a suitable key value, first install your extension from a .crx file (you may need to upload your extension or package it manually). Then, in your user data directory, look in the file Default/Extensions/_<extensionId>_/_<versionString>_/manifest.json. You will see the key value filled in there.