Create issue and set assignee via rest api

[Konrad Poznanski]

Hello, 
I have to implement a REST API call to create an issue in a Jira Service Management project.
The issue type I use has the assignee field visible in both Create and Edit/View screens.
I prepared and used the following request that I'm showing below with masked values.
As you can see, I set the assignee and the issue is created on Jira Service Management, but if I open the issue the assignee is not set.
I took the assignee id searching my user account in the section "People" and copied it from the url of my user account page resulting with the search.

url where I took the assignee id 
https://my.jira.software.manager.domain/jira/people/xxxxxxxxxxxxxxxxxxxxx


url for rest api call:    https://my.jira.software.manager.domain/rest/api/3/issue

request body:

{

    "fields": {

        "summary": "Request Summary",

        "issuetype": {

            "id": "10011"

        },

        "project": {

            "id": "10005"

        },

        "assignee": "xxxxxxxxxxxxxxxxxxxxx",

        "customfield_10065": {"disabled": false, "value": "value1"},

        "customfield_10067": {"disabled": false, "value": "value2"},

        "customfield_10093": {"disabled": false, "value": "value3"},

        "customfield_10066": {"disabled": false, "value": "value4"},

        "customfield_10099": "myvalue1",

        "customfield_10083": "myvalue2"

    }

}

The new created issue but without assignee 


Can you tell me where is my error?
Thank you.