Hello @EE NTU ,
it seems you want to connect your hololens as a device to the IoT Hub.
as seen here, you need to create a DeviceClient:
var connectionString = "[the connection string of your device]";
using var deviceClient = DeviceClient.CreateFromConnectionString(connectionString);
The service client is for controlling the IoT Hub service itself (together with other classes) and it needs the connection string of the IoT hub itself. It's not recommended to expose the IoT Hub connection string outside Azure / inside a device.