Set assignee while transitioning issue - REST API

[Gleb Kartashov]

I'm using /rest/api/2/issue/SD-177128/transitions URL with JSON : 

{
"assignee" : "jadmin",
"transition" : {
"id" : "161"
}
}

but this doesn't work. How do I set this up? Assignee field is present on the transition screen.

[Gleb Kartashov]

Also this needs to be a singe API call AND cannot use "assign issue" post functions due to all API calls being executed from a singe system user