Note prefix

If you create a new score with the new score wizard you can define a pickup measure: https://musescore.org/en/handbook/3/create-new-score#time-signature-pic….
In an existing score right click on a measure->measure properties and change the measure duration: https://musescore.org/en/handbook/3/measure-operations#properties.
Or select the notes/rests you want to remove and press ctrl+del.

You mean that pick-up measure?

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