Create a new CardType, create an instance of that CardType.

Edit the structure of that instance and add {{+some name with spaces | type: Basic}}

Now go to that card... notice the name of the plus card says "Add+some name with spaces" in normal form. Click Add and a small window appears. Now instead of Submit click Cancel.

You are back to the original card. Notice the like that spelled "Add+some name with spaces" now spells "Add some_name_with_spaces"

Clicking Add again will create a card with "_" instead of spaces.