RESTRequest1.Resource:='users/*********/messages/15c*******092a4/modify';
RESTRequest1.Method:=TRESTRequestMethod.rmPOST;
RESTRequest1.Params.Clear;

with RESTRequest1.Params.AddItem do
    begin
      ContentType := ctAPPLICATION_JSON;
      Name:='addLabelIds';
      Value:='Label_1';
      Kind := TRESTRequestParameterKind.pkGETorPOST;
      Options := [poDoNotEncode];
    end;
RESTRequest1.Execute;

{
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "parseError",
    "message": "This API does not support parsing form-encoded input."
   }
  ],
  "code": 400,
  "message": "This API does not support parsing form-encoded input."
 }
}