Add first to the for TextBox2 control.
Then Add the following event TextBox2 . This is going to fire when the text is changed.
Same thing you can do with javascript.
private void TextBox2_OnChange(object sender, System.EventArgs e)
{
string newval;
string sval = TextBox2.Text;
string[] lsval = sval.Split(char.Parse("."));
if(lsval.Length == 1)
newval = lsval[0] + ".00";
else
newval = lsval[0] + "." + lsval[1].PadRight(2, char.Parse("0"));
TextBox2.Text = newval;
}
Success