Hi Sreekanth.
Use the /data/objects service with a RegularResourceLink. The properties you need are WorkItem & Resource, both are an EntityId.
Check the Clarizen Data Model for further information.
Sample Request:
Curl https://api2.clarizen.com/v1.0/services/data/objects/RegularResourceLink -X PUT -d {"Resource":"/user/9f2b8dd1-36e0-4886-807e-9d5fb6c07d32","WorkItem":"/Project/4bfc9995-6a9d-4a30-95a2-c4df57abaa43"}
Response will contain the new RegularResourceLink Id, e.g.:
{
"id": "/RegularResourceLink/9e9e5bb9-779c-4f6b-8477-c1f939c67ab2"
}