To achieve this you should use Bar Settings.

At the bar you want the mute to start, right click and chose Bar Settings from the drop down, alternatively, highlight the bar and press f5 on your keyboard.
Both will bring up the Bar Settings menu.

On this menu,the drop down to the left of the instrument name gives various options, one of which is Mute.

To start the instrument playing again, bring up Bar Settings at the relevant bar and chose Back to Normal.

It would be worthwhile to get familiar with the various Bar Settings options as they can be very useful.

Hope this helps
Vintage