Let's say you would like a lead-in prior to starting recording. You need to set the count-off

!ProTools - Countoff.excalidraw.svg

Click/Count-Off Settings

Go to Setup -> Click/Countoff to access miscellaneous settings concerning the metronome.

!Count Off _0.excalidraw.svg

Some useful settings include the ability to only hear the click during the countoff, the ability to only hear the click during record, and the ability to only hear the click during playback.

!ProTools - Click and Countoff Options.excalidraw.svg