Setting User field using jira API

[Apoorv Jain]

I have been able to find various questions similar to this one, but none of them are solving this problem.

So I have this custom field

 "customfield_10039": {

              "required": false,

              "schema": {

                "type": "user",

                "custom": "com.atlassian.jira.plugin.system.customfieldtypes:userpicker",

                "customId": 10039

              },

              "name": "user",

              "key": "customfield_10039",

              "autoCompleteUrl": "https://integrationtr.atlassian.net/rest/api/1.0/users/picker?fieldName=customfield_10039&fieldConfigId=10140&projectId=10001&showAvatar=true&query=",

              "hasDefaultValue": false,

              "operations": [

                "set"

              ]

            },

So as you can see this allows one to set the user, now I set the user using this:

{"fields":{"customfield_10039" : {"name":"admin"}}}

I have tried so many combinations for name, I have tried using email id, display name, even name by concatenating first name, second name. In each one I get the same error:

{"errorMessages":[],"errors":{"customfield_10039":"user is required."}}