If doing it in Finale, I'd create a percussion track. Assign a different instrument to the downbeat of each measure than the following beats. Add a few measures to the beginning (at least 4) and give a clear countdown in the instructions such as, You will hear 4 measures before the downbeat. The piece begins with the 4th beat upbeat to measure 1... or something like that. Now export your scratch track with the percussion.

I don't do this in Finale, however. I create scratch tracks in a DAW. I give verbal instructions with my click. I actually talk over the intro by recording the click onto one track and talking on the second. "I will give a four bar count down to the Intro. Ready... 4… 3… 2… 1… Intro" If there are pauses, this is the way to do it as you give verbal cues. For a rit., you would stop the click and count the beats in the ritardando. “Ritard, 2… 3…. hold…... tempo… Now." Resume the click for the a Tempo on the Now.