Download the ZIP file from the Releases page (or Direct Download), then go to chrome://extensions and turn on the developer mode toggle, then click load unpacked and right click on the ZIP file then click extract, then click on the folder and then click open. Now you can open it from the launcher.
I used an extension called Extension Source Downloader to download the SecureTestBrowser App source code as a ZIP archive, then I changed line 34 from '"kiosk_enabled": true,' to '"kiosk_enabled": false,' in manifest.json and did the same with line 35