Are synonyms functionallity is supported if index is running on dev find account?

Request:

GET http://es-api01.episerver.com/QkEI9tXN8so30PlQV3TJH9dTtFBeszkJ/yauheni_0663/_config HTTP/1.1

Response:

{
    "status": "ok",
    "item": {
        "stats": true,
        "admin": true,
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "languages": ["no", "pl", "sv"],
        "connectors": false
    }
}

"admin" propety is set to: true.(Repeating "language" property looks like a bug)

I'm able to use UsingSynonyms extension when I do search - there is no error during request:

{
    "query": {
        "query_string": {
            "query": " ... ",
            "analyzer": "synonym_[language]"
        }
    }
}

But if I try to add synonym:

            var synonymsClient = SearchClient.Instance.Optimizations().Synonyms();
            synonymsClient.Add(new Synonym("word1", "s1"), Language.Swedish);

Following request is issued:

POST http://es-api01.episerver.com/QkEI9tXN8so30PlQV3TJH9dTtFBeszkJ/yauheni_0663/_admin/synonym/?

{
    "$type": "EPiServer.Find.Optimizations.Synonyms.Api.Synonym, EPiServer.Find.Optimizations",
    "phrase": "word1",
    "synonym": "s1",
    "bidirectional": false,
    "tags": ["language:sv"]
}

which failes with 403 Forbidden error:

{
    "status": 403,
    "error": "Your key is not authorized to access (POST) '/yauheni_0663/_admin/synonym/?'"
}