Try something like this in the Execute Method of your ICommand
ESRI.ArcGIS.Client.ArcGISTiledMapServiceLayer layer = new ArcGISTiledMapServiceLayer();
layer.ID = "Overview LayerName";
layer.Url = "http://services.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer";
ESRI.ArcGIS.Client.Toolkit.OverviewMap oMap = new ESRI.ArcGIS.Client.Toolkit.OverviewMap();
oMap.Map = MapApplication.Current.Map;
oMap.Width = 100;
oMap.Height = 100;
oMap.Layer = layer;
oMap.MaximumExtent = new Envelope(-41381301, -19971868, 41381301, 19971868);
MapApplication.Current.ShowWindow("Overview", oMap);