public string OrganizationId {get; set;}

{
  var owner = portal.GetSignOnUsername();
  var portalInfo = await portal.GetPortalInfoAsync();

  //1. Get all web maps
  var query1 = PortalQueryParameters.CreateForItemsOfType(PortalItemType.WebMap);

  //2. Get all web maps and map services - include user, organization
  // and "usa" in the title
  var query2 = PortalQueryParameters.CreateForItemsOfTypes(new List<PortalItemType>() {
  PortalItemType.WebMap, PortalItemType.MapService}, owner, "", "title:usa");
  query2.OrganizationId = portalInfo.OrganizationId;

  //retrieve in batches of up to a 100 each time
  query2.Limit = 100;

  //Loop until done
  var portalItems = new List<PortalItem>();
  while (query2 != null)
  {
    //run the search
    PortalQueryResultSet<PortalItem> results = await portal.SearchForContentAsync(query2);
    portalItems.AddRange(results.Results);
    query2 = results.NextQueryParameters;
  }
  //process results
  foreach (var pi in portalItems)
  {
    //Do something with the portal items
  }
}