Insert a note

See: https://musescore.org/en/handbook/4/alternative-note-input-methods#inse…

Turn on insert note entry mode and enter the note. See https://musescore.org/en/handbook/4/alternative-note-input-methods#inse…

You should understand that this will extend the duration of the measure - e.g. a 4/4 measure will end up with 5 beats if you insert a quarter note. If you don't want that to happen you should stay in the default note entry mode but select and then cut the notes following the position where you want to insert a note, add the note you want to insert and then paste back the previously cut notes after the newly inserted note.