q
.Missing(c => c
    .Name("named_query")
    .Boost(1.1)
    .Field(p => p.Description)
    .NullValue()
    .Existence()
)

new MissingQuery
{
    Name = "named_query",
    Boost = 1.1,
    Field = "description",
    NullValue = true,
    Existence = true
}

{
  "missing": {
    "_name": "named_query",
    "boost": 1.1,
    "existence": true,
    "field": "description",
    "null_value": true
  }
}