You are not able to get the text of asp.net textbox controls because when page is being rendered its id got change. You can see using View page source.
There are many ways to get the value of textbox. Easiest way as given below (I am not saying that it is a good practice but for time being you can do).
Modified:
<asp:textbox id="txt_Install" runat="server" name="mytextbox" onkeyup="DueAmt()" xmlns:asp="#unknown"></asp:textbox>
Sample of script code:
function DueAmt() {
var install = document.getElementsByName("ctl00$MainContent$MainContent_txt_Install")[0].value;
alert(install);
}
Here “ctl00$MainContent$MainContent_txt_Install” is taken from View page source.