How to map mac command key in vim?

It is true that <D- maps to command key. You can see it by :help <D-.

However, this will not work in your vim+terminal. It only works in MacVim. See here

The best shot for your intention is to map the combination as hex code, sending to terminal.

Still I will not recommend doing this. command is too important for mac os environment. For example, anyone uses an input method can't map it to command+space.\

Please consider using the usual method here: <leader>.

iTerm2 can do this - here I mapped command enter to :wq

With IdeaVim you can use: <M-*>