iReal b doesn't have a special notation for key changes but you can use the staff text line. For example, "/4/" above a chord could mean that the tonal center of a tune is going up a fourth. This is what I find simplest and easiest to read:

UP DOWN
half-step /h/ \h\
one step /1/ \1\
minor 3rd /m/ \m\
third /3/ \3\
fourth /4/ \4\
tritone /T/ \T\
fifth /5/ \5\