1. Type "foo" into the search bar.
2. Use the down key to move to the "Add new card".
3. Use the up key to go back to "Search for".
4. Hit return, and you get an offer to create the card.

In fact, after 2. you can delete everything in the Search bar, come back to it later and type in foo and hit return, and you'll still be taken to foo instead of searching for it.