You can pin an icon for this purpose:
1. Copy to notepad and save as Show desktop.scf:
[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop
2. Pin an icon to your taskbar, for example notepad.
3. Right-click the pinned icon >> properties >> in the target box type the path to Show desktop.scf >> e.g. C:\Users\Show desktop.scf