Adding a note on the 4 in a 4/4 measure.

this song I'm composing (which I've played an sung for years) is in 4/4 time, but the vocals start in at the 4 beat at the end of the 8th measure. When I select Note input then hit the note I want, it won't take and just puts a rest in. Where do I find the feature to allow inputting the note, without screwing with the time signature?