Summary

There is no official and supported API endpoint to update the board administrators in the Jira Cloud REST API, but there is an available endpoint to do it. 

Atlassian no longer supports this endpoint, so there is no official documentation for it, nor guarantee it will continue working as expected in the future.

Environment

Jira Software Cloud

Solution

To update board administrators, use the following endpoint:

PUT /rest/greenhopper/1.0/rapidviewconfig/boardadmins

curl:

curl --location --request PUT 'https://<instance>.atlassian.net/rest/greenhopper/1.0/rapidviewconfig/boardadmins' \
--header 'Content-Type: application/json' \
--header 'Authorization: Basic <authorizationToken>' \
--data '{
    "id": 2,
    "boardAdmins": {
        "userKeys": [
            "<accountId>",
            "<accountId>"
        ],
        "groupKeys": []
    }
}'

You just need to replace "<instance>", "<authorizationToken>", and "<accountId>" with your data and execute it.

While there's no official documentation, you can check the WADL here.