Hi Nuno,
If you have to send a notification depending on some condition, you should consider to place this logic in your server side.
For example, you can set a timer to run periodically and then send a notification depending on whatever you want.
Anyway if you want to try implement some asynch Client Actions you can try to take a look at this documentation.
It will explain how to run client actions without the need to wait for it response, but afaik you will not be able to run it in background when the app is closed or in second plane for example.
If you need that you can try to use this other plugin to prevent the app from sleeping, if this don't suit your needs I'm afraid you need to write some custom cordova code on a plugin to to what you want.
Check also these other posts to see if they help:
- https://www.outsystems.com/forums/discussion/35157/background-client-action/
- https://www.outsystems.com/forums/discussion/21539/how-to-display-push-notifications-while-app-is-in-background/
Hope it helps,
RR :)