How to create user with role(s) through REST service?
After many trials, I was able to resolve my issue

Here is the JSON format to create user with role.

{
   "name":"user343",
   "pass":"kes35@r4",
   "mail":"user343@sample.com",
   "status":"1",
   "roles":[
      "4"
   ]
}


To assign multiple roles,

{
   "name":"user344",
   "pass":"kes344@r4",
   "mail":"user344@sample.com",
   "status":"1",
   "roles":[
      "3",
      "4"
   ]
}


I hope this helps someone.