How to programmatically pair a bluetooth device on Android
public void pairDevice(BluetoothDevice device) {
        String ACTION_PAIRING_REQUEST = "android.bluetooth.device.action.PAIRING_REQUEST";
        Intent intent = new Intent(ACTION_PAIRING_REQUEST);
        String EXTRA_DEVICE = "android.bluetooth.device.extra.DEVICE";
        intent.putExtra(EXTRA_DEVICE, device);
        String EXTRA_PAIRING_VARIANT = "android.bluetooth.device.extra.PAIRING_VARIANT";
        int PAIRING_VARIANT_PIN = 0;
        intent.putExtra(EXTRA_PAIRING_VARIANT, PAIRING_VARIANT_PIN);
        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        context.startActivity(intent);
    }

Intent intent = new Intent(BluetoothDevice.ACTION_PAIRING_REQUEST);
intent.putExtra(EXTRA_DEVICE, device);
int PAIRING_VARIANT_PIN = 272;
intent.putExtra(BluetoothDevice.EXTRA_PAIRING_VARIANT, PAIRING_VARIANT_PIN);
sendBroadcast(intent);

Intent intent = new Intent(Settings.ACTION_BLUETOOTH_SETTINGS);
    startActivityForResult(intent, REQUEST_PAIR_DEVICE);