graphQL query with variables (C#): what is the correct syntax for the query

I am writing a C# test using this client: https://github.com/graphql-dotnet/graphql-client. Here is my query:

query getFilters($input: FilterInputType!) {
  getFilterData(input: $input) {
    assetId
    material
    productFamily
    equipmentModel
  }
}

And here is the query variables:

{
"input": {
    "startDate": "2019-09-01T00:00:00",
    "endDate": "2019-09-30T23:59:59"
}

}

When I execute the query in RestClient it returns data, but the query in the code does not work. Here is my code

        var startDate = "2019-09-01T00:00:00";
        var endDate = "2019-09-30T00:00:00";
        var dateRange = new { startDate, endDate };
        var getFiltersRequest2 = new GraphQLRequest
        {
            Query = @"
            {                    
                query getFilters($input: FilterInputType!) {
                  getFilterData(input: $input) {
                    assetId
                    material
                    productFamily
                    equipmentModel
                    sourceSite
                    destinationSite
                  }
                }
            }",
            OperationName = "getFilters",
            Variables = new
            {
                input = dateRange
            }
        };

        //Act
        GraphQLClient graphQLClient = new GraphQLClient(endpointUrl);

        graphQLClient.DefaultRequestHeaders.Add("Accept", "application/json");
        graphQLClient.DefaultRequestHeaders.Add("Authorization", authToken);
        var graphQLResponse = await graphQLClient.PostAsync(getFiltersRequest2);

What might be wrong with my syntax?