Click the download button above. Double-click the ZIP file to extract it. You'll get a folder called boycott-alerts-workaround.
In Chrome, go to chrome://extensions/ or click the three dots → Extensions → Manage Extensions
Toggle "Developer mode" ON in the top-right corner
Click "Load unpacked" and select the boycott-alerts-workaround folder
Make sure "Automatically allow access on the following sites" is turned ON (blue)
Visit Elvis Costello on Ticketmaster - you should see a floating alert!