If you mean, you have already put repeat signs around it and added text saying "Play 8X" or whatever", but want MuseScore's playback function to do this as well, right click the measure, Measure Properties, sert the repeat count.
If you mean, you wish to make eight copies of that measure, select it, press "R" seven times.
Comments
Take a look here:
https://musescore.org/en/handbook/measure-operations-0#properties
The box Repeat count
If you mean, you have already put repeat signs around it and added text saying "Play 8X" or whatever", but want MuseScore's playback function to do this as well, right click the measure, Measure Properties, sert the repeat count.
If you mean, you wish to make eight copies of that measure, select it, press "R" seven times.