my hunch is you need to lose the focus on the input element in order to Fire the onChange event. There are many ways to switch the focus inside a browser instance. The easiest way is probably to record a Tab click. Since you seem to be using the WebAii Framework and not WebUI Test Studio itself, here the code that will let you do that:
), 150, 1);
Try adding this line after TxtName.Text = "TestValue".
Let me know how it goes!
All the best,
the Telerik team