MuseScore -- Index -- Hairpins

Hairpins are line objects.

To create a hairpin first select a note to mark the start point.

<H>   create a crescendo hairpin
<Shift+H>   create a diminuendo hairpin (decrescendo)

Haipins could also be created by dragging a hairpin symbol from the line palette to a note head.

<H> create a crescendo hairpin:
<double click> switch to edit mode:
<Shift+right> move end anchor:
<right> move end point: