Yes, this also never worked for me
My guess it is working because the app not really is in the background...
To find out, if it really is working in the background, you have to close the app using the close application block
@Hassan_Antonios_Saad follow the advice from @TIMAI2
Taifun
Yes, I will try to fix that on my device sittings, I used to do that but I hadn't got anything, I'll try again, I don't matter which is the device because I want to hide this popup service provider notification even in any device I'll use the application on it, but about my application it works properly and I'm so sure it on now on my device, it had sent the SMS I programed to and I saw that on my device, the messages had been sent as I want to, you can try the project aia I'd sent to you, but you need to make 3 files on your internal memory, one for names and number and the other for the messages sittings and the last one for ordering the number by the date .. my application works properly in background even when I reboot my device or when I close the application, and the icon appear on the status bar clearly.
Thanks a lot, but in fact I need an extension or something like this to make the device stop popping up this service provider notification when the application send SMS in background, then get the situation as it was before sending the SMS I mean to turn the sittings of the device as it was before sending the SMS, thanks a lot for your interest and help.
Your service provider is sending the notification to your phone/device (not the AppInventor app).
As previously advised, either:
Yes, I've got it, the Service Provider is who send the popup notification not the Application, I know, and I guess it isn't an easy way to contact to the Service Provider to tell them to stop that, I'd tried to find something on the device sittings like to stop the notification for example but I didn't reach a way to make the device stop that for me, yes I did it I'd blocked the notification on my device but this popup notification still appear after sending the SMS, but as a application inventor we can control our devices and make them do what we need them to do I guess, I need to control the device and ask it to do what I want from him to do, thanks a lot.
We all dream for this too 
Do you not have any settings you can change in your account with the Service Provider?
(Who are they ?)
I guess it is a little complicated thing, but I'll try to find if I can stop this service provider popup sms notification, but also I need an extension to program that on my device
And what extension would that be? Unless we know the service provider, and whether they have configurable options, we can progress no further...
Anyway, in your device when you send an SMS, is there no popup notification from your service provider that tell you something like that ? didn't you use a prepaid service for example, thanks a lot for your interest and I appreciate that.
No
Taifun
Not for me either.
It's not my business I want that extension from you, even your devices don't send like this pop up service provider notification, aren't we friends ?
Hey .. creative persons can create every thing they want (or their friends want) .. you can invent like this extension .. please help me.
Just invent extensions for me and I'll invent creative application because of you !
I use Google's Messages for my SMS.
It offers me the option to turn off all notifications:
Maybe you have this setting for your SMS provider.
I will check it, thanks a lot, and I'll tell you if I got it
@Hassan_Antonios_Saad Have you ever tried to activate the "Not Disturb Mode" on the device BEFORE sending the SMS?
If this works, I believe that there must be a way to activate the Not Disturb Mode programatically, for a while (using Clock) before sending the SMS, so that it is disabled later, in order to prevent notification, which should arrive a few seconds after the SMS submitted...
"When Do Not Disturb mode is enabled, your phone will silence calls, alerts, and notifications, and your screen won't light up. You can create custom schedules for Do Not Disturb, and allow exceptions for certain sources, such as alarms or messages from people on your Favorites list."
Also try putting the device screen down on the table as soon as the SMS is sent to see if the notification arrives anyway...
https://www.youtube.com/results?search_query=How+to+Activate+The+"Do+Not+Disturb"+Mode+on+Your+phone
Lito
@>-->---
I will try it, thanks a lot
'Do Not Disturb (DND) state' is not allowed for third party apps... but if it is for personal use I think it can work... 
Lito
@>-->---
OK, I appreciate that for you, thank you very much.