How do I find the cursor position in the box ?

Question

Hi,

I'm making phone simulation with asp.net. It is like the screenshot links  ( http://prntscr.com/6l4q8c ). 

 I used the control of Asp.net. I would like to be added to the location of the cursor on the new number.

 But I do not know the position of the cursor. How do I find the cursor position in the box ?

 

 Active block of code is the same as the following

 

if (txtNumber.Text.Length > 0)

{

txtNumber.Text = txtNumber.Text.Substring(0, txtNumber.Text.Length - 1);

}

else

{

txtNumber.Text = string.Empty;

}

 

 

Thank you for your help