I usually do it this way…
(lets say I want a 1 bar count-in before the start of the new tempo)…
I create a MIDI track, routed to a metronome sound, and record one bar of click during the first bar of the new tempo.
I then switch that MIDI track over to Linear Timebase, set Snap to “Events”, then drag-copy that one bar click Part backwards, such that the end of the copied Part butts against the start of the original. (If necessary I can then delete the original, although I am more likely to want to keep it).