It cannot be done directly from Synapse settings, but you can do it manually by editing Synapse configuration.

1. Close Synapse
2. Edit ~/.config/synapse/config.json
3. Set activate to Super_L

{
  "ui" : {
    "shortcuts" : {
      "activate" : "Super_L",
      ...
    }
  }
}

There is a disadvantage of this solution. You basically can’t use any other global key combinations with Win key, because it will always trigger Synapse.

If that is a problem you can use xcape to assign an additional one-click function to a modifier key.

xcape -e 'Super_L=Super_L|space'

There is a handy feature in Windows, when you type Win+. a selector for Emoji and other Unicode characters will appear.

You can install a similar app in Arch Linux using gnome-characters package.

sudo pacman -S gnome-characters

Now you can mimic the Windows feature and assign a global hotkey for Win+. to execute gnome-characters.

Using xinit you can start applications without even starting a window manager. There is a sample in Arch Linxux wiki, but you may notice that Chromium does not fill full screen.

With the example below a specific site is started in kiosk mode taking whole screen. Change the size based on your display.

exec chromium --app=https://example.com --kiosk --window-position=0,0 --window-size=1366,768