You can use Quick play option if you want to play a composition on a display right now for a specific time. Quick play option takes a higher precedence than already existing schedule in a display. This should be ideally used for emergency notification.
Create a composition if you do not have created one.
Move your mouse to that composition to see the "Play" icon and click on it.
Give a name to this quick play and provide time till when the content should run.
Select the screen/s on which you would like to assign this quick play.
Click on "Submit". Your contents are now live on the selected screens.