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/content/EXTENSION_ID/EXTENSION_VERSION/manifest.json. You will see the key value filled in there.