Hi Sumit,
if it is about sending a String/JSON to a device, the Shell command might fit best in this case. It already comes with a built-in SmartREST template. You just need to subscribe to the topic s/ds for your device to receive a shell operation.
You need to first announce the capabilities of your device to Cumulocity as described in the guide. This means, you need to tell Cumulocity that your device actually supports shell command operations. You can use the static template 114 for this to publish a message on the topic s/us. For example: 114,c8y_Command.
Once you have done this, the Shell tab should appear for your device.
You can also follow the discussion here as it is the same topic.
Best regards
Christian