hi Harshraj Singh,
can you post the error code or picture which you are getting in postman?
Have the given the content type as application/json in Postman?
It looks like you are using the base url (https://demptestapplication-sandbox.mxapps.io/rest/myservice/v1) as an endpoint, but you need to add the resource '/resource' as well.
So using the endpoint https://demptestapplication-sandbox.mxapps.io/rest/myservice/v1/resource should work in Postman.
Also look at the Postman documentation of importing a swagger.json, this will immediately create a working GET call (use the URL for your swagger.json: https://demptestapplication-sandbox.mxapps.io/rest-doc/rest/myservice/v1/swagger.json).
Hope this helps.