Follow up, Follow-up, or Followup - Which is Correct?

I've been wanting to follow up on the follow-up I've been intending to do to figure out which of these three ways of writing this word/words is correct.  (Hint: I used them correctly in the preceding sentence.)
- follow up
- follow-up
- followup

I searched and found the following link that explains it all quite simply:
Follow Up, Follow-Up, or Followup

The short of it is that "follow up" is a verb and "follow-up" is a noun or an adjective, and "followup" is not a proper or valid way to write it.

A simple rule for knowing which to use, is if you can use the word the in front of it, then it should be follow-up (with a dash).  Otherwise, without the in front of it, it should be follow up (with a space).