Notes for concert castanets

1) add castanets to your score, then right click into the staff, staff properties, make it 2 lines. Them (pretend to) enter notes, edit drumset to add the other hand's castanet.
2) now save that score in your templates directory

Wouldn't it be better: https://musescore.org/en/handbook/3/drum-notation#sticking ?

In the instrument dialogue (by creating a new score or after it with edit->instruments...) exist castanets as instrument, switch from "common instruments" to "all instruments" or type it in the search field. But only with one note line. Maybe it makes sense to add it twice - the one for the right, the other for the left hand? You can also add the second staff to the existing instrument inside the instrument dialogue.
Else, for editing the lines of an instrument you'll find it with right click on this instrument->staff/part properties...
Not really sure, if it makes sense, but via right click you also can edit the drumset.
Of course you can save the result as template in the template folder of MuseScore.

A staff is the set of five parallel horizontal lines that you write music on - or when writing for most percussions instruments, the single horizontal line.