Given Shane’s answer, I’m not sure that I correctly understood your question.
Given my understanding :
Enter the System prefpane Notifications (here is its french name) and select the 3rd “Style d’alerte”, which is named “Alertes” in French.
With such setting you will get notifications remaining on the screen until you trigger one of the two buttons available.
Yvan is correct of course – I meant you can’t control it from the script. And to control it in System Preferences, I think the app has to be code-signed.
Ok, I think I understand. If I save the Applescript as an app and code-sign it, it’s alerts will automatically change to have a close and snooze button?
No. You also have to make a change to the app’s Info.plist file to make it the default for the app, and hope the user doesn’t change things in System Preferences. If the user chooses banner style instead, there’s nothing you can do about it – it’s their choice.
If it’s just for yourself, you can make the change in System Preferences and skip the Info.plist change.