Typically the Mapped Key Wizard is enough, in addition to what Khatovar explained about the Hold option. This is a check box in the Mapped Key Wizard for next time.

For your existing Mapped Key, it's easy to fix. In the top left pane under Key Maps, select the one you put the Space mapping under. Then in the bottom left pane, select your Space mapping. Then in the bottom right pane is a "Hold any Keystroke Actions while Hotkey is held" option, set that to On!