How to insert numbers into an already populated TextBox in a RadNumericUpDown

I am working on testing passing keystrokes into a TextBox within a RadNumericUpDown control that is part of my WPF application. I am trying to insert a digit into a number already in a TextBox. Consider the following situation...

I have just typed the value 1000 into the TextBox while recording. Test Studio records my left mouse click that focuses in on the TextBox in a single step and records the typing of 1000 in the proceeding step. Now, I move focus away from the TextBox I just entered 1000 into by hitting the tab key.

I would like to add the digit 1 into the 1000 already present in the TextBox such that the value ends up being 10(1)00 or 10100. So I focus my cursor at the position in the TextBox between 10 and 00, type 1, hit tab, and set up a verification step to make sure the value in the TextBox has turned into 10100. Visually at this point in the recording, I see 10100 in the TextBox.

I then quit recording my application so I could execute it. When I execute it, it fails on the verification step because when I enter the 1 into the TextBox, it replaces the entire TextBox value from 1000 to 1. So instead of having 10100 in the TextBox, it has 1.

Is it possible for me to insert a digit into a TextBox value without bombing on the rest of it?

*It should be noted that I am simulating both real keystrokes and mouse clicks (not sure if this matters).*

Thanks!

~ Max