If you click in the blue space above the note you're trying to alter and drag to the right (highlighting the section), you'll now find you can drag the yellow line down. This is equivalent to adding a pair of nodes at either end of the selection.
Once you've done this, go into the Formant page, make the same selection (if it isn't already highlighted) and drag the line UPWARDS by a small amount to compensate for the drop in pitch.