Install libappindicator
$ sudo pacman -S libappindicator-gtk3
Modify the desktop file (/usr/share/applications/slack.desktop)
Add to the exec env
XDG_CURRENT_DESKTOP=Unity
Example
Exec=env XDG_CURRENT_DESKTOP=Unity LD_PRELOAD=/usr/lib/libcurl.so.3 /usr/bin/slack %U
thanks!