Adding a custom field.

searchResults.cs

[TestMethod]
public void Search_Results()
{
  var searchTuningResource = new Mozu.Api.Resources.Commerce.Catalog.Admin.SearchResource(_apiContext);

  var settings = searchTuningResource.GetSettingsAsync().Result;

  settings.SiteSearchSettings[0].CustomFields.Add(
      new SiteSearchRelevancyCustomField() 
          { 
              FieldName = "tenant~best-use",
              FieldWeight = 3
          } 
      );

  var result = searchTuningResource.UpdateSettingsAsync(settings).Result;
}