We don't yet have an official Extension on the Chrome Webstore, so in the meantime to use Extendify on Chrome, you can build it yourself and then use it from your local filesystem.
You must first build Extendify with the --platform=browser flag enabled. To learn how to do so, refer to the mod build guide.
In your browser, navigate to chrome://extensions.
If you don't have developer mode enabled, please do so.
Continue by clicking Load Unpacked and navigating to the packages/mod/dist folder. (This dist folder is also explained at the end of the build guide)
You can now refresh Spotify or open a new Spotify tab and you should see your webpage change!