Pickup measure

If I understand correctly, you probably want to split a measure so some of its beats are at the end of one line and the rest are at the start of the next. So click the note you want to start the next line and use Tools / Measure / Split Measure Before Selected Note/Rest. Then add a system break on the first part of the bar by selecting it and hitting Enter.

If you mean something else, like maybe a pickup to start a movement, then Measure Properties to set its duration.

Either way, also use Measure Properties to set the pickup to exclude from the measure count.

Right-click into the measure, Measure properties, then change the actual duration and tick the box" Exclude from Measure count"