Attempt of using $link operation in XSOData leads to error. Why?

Hello SDN!

This is my XSOData service:

service namespace "developers.kulaev.indexes.services.odata" {    "developers.kulaev.indexes.db::DATA.IND.ZLIB_IND_BASE" as "IndexBase"    navigates("Index_Busas" as "IndBusas")    create events(        before "developers.kulaev.indexes.js.ue:ind.xsjslib::createIndRecBE"    )    delete forbidden;        "developers.kulaev.indexes.db::DATA.IND.ZLIB_IND_BUSAS" as "Busas"    delete forbidden;        association "Index_Busas"        principal "IndexBase"("ID") multiplicity "1"        dependent "Busas"("IND_ID.ID") multiplicity "*"    update events (before "developers.kulaev.indexes.js.ue:ind.xsjslib::linkIndBusa" )    delete forbidden;
}

ZLIB_IND_BASE has one key field ("ID"), which generates with create event createIndRecBE. ZLIB_IND_BUSAS has two key fields - ID from ZLIB_IND_BASE and ROW_NUMBER. During creation I have to generate key value for ZLIB_IND_BASE (done), fill ROW_NUMBER with counter values (don't know how to realize it in user-exit; done in UI) and set this key for created record(s) in ZLIB_IND_BUSAS via using the $link operation.

I created the followed request:

--batch
Content-Type:multipart/mixed;boundary=changeset
Content-Transfer-Encoding:binary
--changeset
Content-Type:application/http
Content-Transfer-Encoding:binary
Content-ID: 1
POST IndexBase HTTP/1.1
Content-Type: application/json
Content-Length:82
{"MANDT":"300","ID":"","RQST":"X","IND_CODE":"","NAME":"demo123","SHORT_NAME":"a"}
--changeset
Content-Type:application/http
Content-Transfer-Encoding:binary
Content-ID: 2
POST Busas HTTP/1.1
Content-Type:application/json
Content-Length:45
{"IND_ID.ID":"","ROW_NUM":0,"BUS_AREA_ID":15}
--changeset
Content-Type:application/http
Content-Transfer-Encoding:binary
PUT $1/$links/IndBusas HTTP/1.1
Content-Type:application/json
Content-Length:12
{"uri":"$2"}
--changeset--
--batch--

IfI use PUT $1/$links/IndBusas HTTP/1.1 then I get an error:

HTTP/1.1 405 Method Not Allowed
Content-Type: application/json
Content-Length: 168
{ "error": { "code": "", "message": { "lang": "en-US", "value": "The URI is not valid for PUT operation. The URI must point to a single resource for PUT operations."}}}

If I use POST $1/$links/IndBusas HTTP/1.1 then I get an error:

HTTP/1.1 400 Bad Request
Content-Type: application/json
Content-Length: 95
{ "error": { "code": "", "message": { "lang": "en-US", "value": "Modification not possible."}}}

How can I resolve this problem?

Regards,

Lev