Unfortunately, you cannot use CoreBluetooth APIs to attach data to advertisements. On iOS the CBAdvertisementDataServiceDataKey is read-only. While Bluetooth LE allows attaching service data , Apple effectively disallows 3rd party apps from doing this.
You do have a few options:
Encode your data inside a 128-bit service UUID and advertise that. You will need to reserver a byte or two in the UUID to know that it is "your" advertisement, and therefore OK to decode the data from the other byes. This full UUID will only be advertised when your app is in the foreground visible on the screen. Let it go to the background or the screen turn off, and it will no longer advertise in that form. Similarly, receiving iOS devices must also be in the foreground with the screen on. This is because iOS disallows getting background scan results without specifying the matching service UUID up front. And because you are dynamically manipulating some of those bytes, you don't know what it will be.
Do a similar kind of encoding with the 4 byte major and minor fields inside the iBeacon BLE advertisement using CoreLocation. Again, this allows you to transmit only when the app is in the foreground. Receiving, however, can happen to a limited degree in the background (for 5-10 seconds after one of your beacons first appears when combining monitoring and ranging APIs.) The big disadvantage is you only have four bytes to work with.
Advertise data by manipulating the 128-bit background BLE Overflow Area Advertisement. This technique is more advanced, but advertising works in the background. Receiving works in the foreground, and partly in the background -- you can receive if the screen is at least turned on. You can read more about this technique and access free sample code in my blog post herehttp://www.davidgyoungtech.com/2020/05/07/hacking-the-overflow-area.
