Hello Fede,

You can use a style for the GroupByRecordPresenter and set the ContextMenu in the style, like this:

      <ContextMenu x:Key="contextmenu">

            <MenuItem Header="Expand"/>

            <MenuItem Header="Collapse"/>

        </ContextMenu>

        <Style TargetType="{x:Type my:GroupByRecordPresenter}">

            <Setter Property="ContextMenu" Value="{StaticResource contextmenu}"/>

        </Style>

You can also achieve this dynamically in procedural code, which you can see how to do here