If you use Slack in a Browser, change the user agent to a Mac OS X one, for example:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3188.0 Safari/537.36

You can use the User-Agent Switcher plugin for chrome. (Their default OS X Chrome is a too old version, so slack won’t load).

If you use the Desktop App it’s a bit more complicated:
Quit slack.

npm install -g asar

Open a console in C:\Users\{your user}\AppData\Local\slack\app-3.0.5\resources
(version number may change in the future)

asar e app.asar app-extract

open app-extract/package.json
Add the whole user agent from above in to the the version field (Keeping the version at the start)
example: “3.0.5 Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3188.0 Safari/537.36”.

move app.asar app-original.asar
asar p app-extract app.asar

And restart slack! You’ll have to re-do this every time slack updates.