so301000.aspx

<ActionBar>
                            <CustomItems>
  <px:PXToolBarButton Text="Insert Row" SyncText="false" DependOnGrid="grid" ImageSet="main" ImageKey="AddNew">
    <AutoCallBack Target="ds" Command="AddNewDSLine" />
    <ActionBar ToolBarVisible="External" MenuVisible="true" /></px:PXToolBarButton>
    </CustomItems>
                                </ActionBar>

SOOrderEntry_Extension.cs

namespace PX.Objects.SO
{
  public class SOOrderEntry_Extension : PXGraphExtension<SOOrderEntry>
  {
    #region Event Handlers

  public PXAction<SOLine> AddNewDSLine;
    [PXButton(CommitChanges = true)]
    [PXUIField(DisplayName = "Insert Line", Enabled = true, MapEnableRights = PXCacheRights.Select,
      MapViewRights = PXCacheRights.Select)]
    protected virtual IEnumerable addNewDSLine(PXAdapter adapter)
    {
      return adapter.Get();
      }
    #endregion
  }
}