Hope this can help:
In the piano roll, there's a chord selector (top-right below the cross). For all the major chords, select major and then click on the root notes (i.e. D#, G#, F, C). For the minor chords, select minor and click to add the root notes for the minors (i.e. A#m, Fm).
You could do it manually as well, but it's more effort. If you want to do it manually, the major chords are the first, third, and fifth note of the major scale of the root note. Minor chords are the first, third, fifth of the minor scale.