@Display Name asked about using SortOptionsDescriptor instead of just SortOptions. There is another parameter for sort that takes an

 Action<SortOptionsDescriptor<TDocument>>[] 

for sort options.

You can then just create something like this and pass it to the Sort of a search

Action<SortOptionsDescriptor<TDocument>>[] sortoptions =
[
    sd => sd
        .Field("_score", SortOrder.Desc)
    , sd => sd
          .Field(f => f.Counter, SortOrder.Desc)
];