Establishing BLE pairing by passkey method

Question

I'm able to connect to the Bluetooth device, but iOS doesn't ask for pairing with it in result bluetooth icon keeps blinking on the connected device's screen. I found it required that passkey should be transmitted and iOS would take care of that automatically once the correct argument in place.

func centralManager(_ central: CBCentralManager, didConnect peripheral: CBPeripheral) {
        if peripheral == self.peripheral {
            print("Connected to your Bluetooth Device")
            peripheral.discoverServices([Peripheral.bloodPressureMeasurementUUID, Peripheral.bloodPressureMeasurementUUIDSecon, Peripheral.currentTimeUUID, Peripheral.currentTimeServiceUUID])

        }
    }

I'm currently using these delegate methods: centralManagerDidUpdateState didDiscover didConnect didDiscoverServices didDiscoverCharacteristicsFor

But can't figure it out in which function argument I should pass that pairing should be established by a passkey?