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.