1- Go to your Apple Developer account and sign up.
2- Click on the “Certificates, Identifiers & Profiles” section.
3- On the left section, click on “Identifier”, add a new identifier and select the “App IDs” option. Click on continue.
4- For the next step you need to write the Bundle ID.
Then enable the "Apps Groups" "Associated Domains" "Push Notifications" and "Sign In with Apple" below. Click "Continue".
5- After you made this you can click on the "Register" Button and the process will be saved.
6- Repeat the same process and create another Identifier, but this time add ".OneSignalNotificationServiceExtension" at the end of the Bundle ID.
7- Once you have registered the two identifiers, you will need to register a new App Group.
8- On the section of the identifier name, make sure to write the same name as the first identifier and end it with ".onesignal" (The "group" at the beginning is set up automatically).
9- Once you have done this, go to the identifier section and select your new identifiers.
Enable the new App Group that you have created in each one and that's it. The process will be complete.
Now, you can build your apps and receive the push notifications in your iOS devices.