anacrusis bar

Take a look at the Handbook (https://musescore.org/en/handbook/4). I recommend reading it through from beginning to end. Don't try to read to actually learn it all! Instead, skim through it as quickly as you can. This will give you the big overview of what's in the Handbook. Alternatively, a skim through forum titles for the past week or so would have probably found this same answer, since this question gets posted about three times a month. :-)

For your immediate question, see https://musescore.org/en/handbook/4/pickup-and-non-metered-measures