This must be installed in /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js use the editor of your choice to replace the contents of ssb-interop.js with the code below.
I use Sublime Text, and called it from CLI via;
sudo subl /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js
Note: it's a very good idea to make a back up copy of ssb-interop.js
before making these modifications.