public Envelope Extent {get; set;}

//namespace ArcGIS.Desktop.Mapping.Offline
var extent = MapView.Active.Extent;
var map = MapView.Active.Map;

//await if needed...
QueuedTask.Run(() =>
{
  //Check map has sync-enabled content that can be taken offline
  var hasSyncEnabledContent = GenerateOfflineMap.Instance.GetCanGenerateReplicas(map);
  if (hasSyncEnabledContent)
  {
    //Generate Replicas and take the content offline
    //sync-enabled content gets copied local into a SQLite DB
    var gen_params = new GenerateReplicaParams()
    {
      Extent = extent, //SR of extent must match map SR

      //DestinationFolder can be left blank, if specified,
      //it must exist. Defaults to project offline maps location
      DestinationFolder = @"C:\Data\Offline"
    };
    //Sync-enabled layer content will be resourced to point to the
    //local replica content.
    GenerateOfflineMap.Instance.GenerateReplicas(map, gen_params);

  }
});