Found on some forum:

This the way to setup the Winkey:
1. K menu > Control Center.
2. Regional & Accessibility > Keyboard Layout > Xkb Options
3. Check the boxes next to “Enable xkb options”, “Third Level Choosers”& “Press any of the Winkeys to choose 3rd level”
4. Press Apply.
5. Regional & Accessibility > Keyboard Shortcuts > Global Shortcuts
6. Scroll down to Panel > Popup Launch Menu
7. Choose Custom & then press the Win key. When you do, it will labeled ISO_Level3_Shift.
8. Press Apply & close the Control Center.
9. Try the damn Winkeys