q
.SpanFieldMasking(c => c
    .Name("named_query")
    .Boost(1.1)
    .Field(p => p.Name)
    .Query(sq=>sq
        .SpanTerm(st=>st.Field(p=>p.Description).Value("dolorem"))
    )
)

new SpanFieldMaskingQuery
{
    Name = "named_query",
    Boost = 1.1,
    Field = Infer.Field<Project>(p => p.Name),
    Query = new SpanQuery
    {
        SpanTerm = new SpanTermQuery
        {
            Field = Infer.Field<Project>(p => p.Description),
            Value = "dolorem"
        }
    }
}

{
  "field_masking_span": {
    "_name": "named_query",
    "boost": 1.1,
    "field": "name",
    "query": {
      "span_term": {
        "description": {
          "value": "dolorem"
        }
      }
    }
  }
}