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)

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

H create a crescendo hairpin: One-measure crescendo
double click switch to edit mode : Crescendo hairpin in edit mode
Shift + right move end anchor: Crescendo extended to the last note of the next measure
right move end point: Crescendo visually extended beyond anchor point