How do you make app inventor run in background?

Is it possible to make app work in background? I want to make a notice, when the countdown goes to zero, then the notice screen pop out. To make this goal, we want to make the app work in background first. However, the method we found on google, there are lots of resources about "sensors->application message" button, but we couldn't find it, we guess is the version problem. Is there any other solution to deal with it? We will really appreciate it.

Please search the community. This has been asked many times before on how you can make an app work in the background.

@dgeadrey890808 in riferimento al background..@Taifun ha creato una preziosissima estensione a pagamento; che permette di far risvegliare qualsiasi app..ogni volta che si desidera.