I achieved this as follows.
On the form, I had the dedicated input box labelled as "Approximate Scale:"
First register the event "onZoomEnd" on the map as follows:
dojo.connect(map, "onZoomEnd", showMapScale);
The JavaScript function "showMapScale" is shown below:
function showMapScale(extent, zoomFactor, anchor, level) {
var scale = tiledMapService.tileInfo.lods[level].scale;
dojo.byId("scaleText").value = "1:" + scale;
}
Here tiledMapService is the ArcGISTiledMapServiceLayer displaying the base map.
tiledMapService = new esri.layers.ArcGISTiledMapServiceLayer(primaryMapServiceURL);
This way, you can display the scale.
Not exactly the way you want but hope this helps.