For example this one http://stackoverflow.com/questions/1407422/code-golf-seven-segments
You can't; only moderators can. This was done because some users would erroneously mark their questions as CW without knowing what it's for.
Also, even more people seemed to think that it should be used for off topic questions. This is not true and off topic questions are not allowed, CW or not.
Long story short the vast majority of the users didn't know what it was for or thought they did, so 90%+ of the questions tagged as CW shouldn't have, and now this functionality is mod only.