"Windows button" on a Mac keyboard

I have been asked by the support department for one of the Windows apps that I run under Parallels to reset their software to fix an issue. The reset instructions begin by telling me to press "the Windows button and the letter R" to open a dialog box, but I have no idea what keyboard button corresponds to the Windows button on my Mac keyboard. I tried the keyboard icon at the top right of the Parallels window but there is no Windows button listed there (although there is a Win + Q entry). Can anyone tell me which Mac keyboard button is the Windows button?

On my wife's Windows keyboard the bottom left buttons are, in order, Control, Fn, Windows, and Alt while on my Mac keyboard they are,, again in order, Fn, Control, Option and Command, and logically I suppose it would be either the Option or Command keys, but pressing any of those with the R button does not open the dialog box that support says should open. If it makes any difference I have Windows 8.1 running under Parallels 13 on a Mac Mini (i7, 16GB RAM, 256GB ssd, High Sierra).

Does anyone know which key is the correct one?

UPDATE: Never mind.

I still do not know which key is the "Windows button" but I decided to assume that all the instructions were telling me to do was to open the Windows Start menu and that assumption appears to have been correct as I can now follow the remaining instructions. I would still like to know, but it is no longer as important as it was when I posted this question.

 

Command key in Mac is Windows key in Windows.

 

OK. Thank you, but is that not supposed to open the Windows start menu? It does not do that on my machine. In fact it does nothing.