Hi @yasminSarbaoui ,
Could you please share a bit more about your scenario?
Do you want to read a message right after it is posted on your teams? And want to get a PowerApps Push notification when a message is posted on your Teams?
If you want to get a PowerApps Push notification when a message is posted on your Teams, I afraid that there is no way to achieve your needs in PowerApps currently. As an alternative solution, I think Microsoft Flow could achieve your needs.
I have made a test on my side, please consider take a try with the following flow's configuration:
When a new channel message is added within your Teams, above flow would be fired, and a notification would be sent to you, when you click the PowerApps Notification within your Mobile device, it would open your current app, then you could retrieve messages from your Teams.
Note: Currently, PowerApps notification is only supported in Mobile device and PowerApps Desktop. You could not get the notification message when you run your app in web browser.
More details about send a PowerApps Notification from Microsoft Flow, please check the following article:
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/add-notifications
In addition, you could also consider add a "Send an Email" action instead of the "Send Push Notification" action within your flow to send notification message:
Best regards,