Hello Sonu Kapoor,
I've answered your questions in the support ticket you submitted. For reference, here's a summary:
1. Set the Owner property of the persistent class (inherited from Content). It contains the ID of the CMS membership user, who is the author of the item. For user interface through which you want to select the author, create a custom field control and use Telerik.Sitefinity.Web.UI.Backend.Security.Principals.UserSelector.
2. Create a custom field as described here.
3. Same as 2), but substitute the content type when calling the service.
the Telerik team
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