I am using Zephyr Cloud Rest API. And unable to assign user to execution(s)

The following call made and received 200 responses. However, I do not see user has been assigned successfully.

POST : 

https://prod-api.zephyr4jiracloud.com/connect/public/rest/api/1.0/executions/bulkAssign
Body: 

{

    "executions": [

        "here execution Id goes"

    ],

    "assigneeType": "assignee",

    "assignee": "here I used accountId for a user from Jira"

}