In order to use MOD_UI left (without combining it with the "r" key, which is used for run on windows) use
DigiKeyboard.sendKeystroke(0xe3, 0x08)
Which uses 0xe3 as key (GUI_LEFT) and 0x08 as modifier (GUI_LEFT_MODIFIER).
This is usesful on systems, where the GUI key is used without combining it with additional keys.