public KnowledgeGraph GetDatastore()

var kgLayer = MapView.Active.Map.GetLayersAsFlattenedList()
        .OfType<KnowledgeGraphLayer>().FirstOrDefault();


QueuedTask.Run(() =>
{
  // use the layer directly
  KnowledgeGraph datastore = kgLayer.GetDatastore();
  
  // or you can use any of the sub items since
  //KnowledgeGraphLayer is a composite layer - get the first 
  // child feature layer or standalone table
  var featlayer = kgLayer?.GetLayersAsFlattenedList()?
                  .OfType<FeatureLayer>()?.FirstOrDefault();
  KnowledgeGraph kg = null;
  if (featlayer != null)
  {
    using (var fc = featlayer.GetFeatureClass())
      kg = fc.GetDatastore() as KnowledgeGraph;
    //TODO use KnowledgeGraph
  }
  else
  {
    //try standalone table
    var stbl = kgLayer?.GetStandaloneTablesAsFlattenedList()?
                    .FirstOrDefault();
    if (stbl != null)
    {
      using (var tbl = stbl.GetTable())
        kg = tbl.GetDatastore() as KnowledgeGraph;
      //TODO use KnowledgeGraph
    }
  }
});

var kgLayer = MapView.Active.Map.GetLayersAsFlattenedList()
                .OfType<KnowledgeGraphLayer>().FirstOrDefault();
if (kgLayer == null)
  return;

QueuedTask.Run(() =>
{
  // get the datastore
  var kg = kgLayer.GetDatastore();

  // now submit a search or a query
  // kg.SubmitSearch
  // kg.SubmitQuery
});