Palm mute line.

P.M.------------|

Palm mute. Guitar technique.
Simple. Line above a note or group of notes. This is a muted version of the note but preserving the tone, produced by placing the right palm on the strings near the guitar bridge.

It's not the same as X head, which means that you mute the string with your left hand thus producing a noise instead of a tone.

You can use text tool, or even an edited line to make it, but it would be good to have it in the lines section, and changing the sound of notes as well.