Assign an icon to an xdg user-dirs.dirs defined directory

["Sr." Ninguém]

Hi,

I'd like to create a custom XDG_CUSTOM_DIR directory in my home
directory pointing to $HOME/Custom and assign an icon to it.

I've done:

xdg-user-dirs-update --set CUSTOM ~/Custom

So far so good... ~/.config/user-dirs.dirs has been changed acordingly.

Now: how do I assign a custom icon to it?