I used to connect the plugin OneSignal like this

How connect the plugin in new format with plugins?

>Solution :
Refer to this answer and one signal documentation we can do that:
plugins { // Project level
...
id 'com.onesignal.androidsdk.onesignal-gradle-plugin' version '0.14.0' apply false
}
then:
plugins { // App/Module level
...
id 'com.onesignal.androidsdk.onesignal-gradle-plugin'
}
...
dependencies {
...
implementation("com.onesignal:OneSignal:4.7.3")
}
To check plugin correctly added refer to answer: 
FYI OneSignal Gradle Plugin releases, OneSignal
Android SDK releases