maintain tabindex after postback in c#.net

What actually i want is,after selecting 1st dropdown list next when i click tab button then cursor needs to go next dropdown list

function fun() {

var txt = document.getElementById("txtid");
if (txt.value == "") {
alert("please enter user Id");
return false;
}
if (txt.value.length <= 12 && txt.value.length >= 5) {

}
else {
alert("must enter min 5 and 12 charachters");
return false;
}

var txtpwd = document.getElementById("txtpwd");
if (txtpwd.value == "") {
alert("please enter password");
return false;
}

if (txtpwd.value.length <= 12 && txtpwd.value.length >= 7) {

}
else {
alert("must enter min 7 and 12 charachters");
return false;
}
var name = document.getElementById("txtname");
if (name.value == "") {
alert("please enter name");
return false;
}
var address = document.getElementById("txtaddress");
if (address.value == "") {
alert("please enter address");
return false;


}
var ddlItemtype = document.getElementById("ddlcountry");
var ddlitem = ddlItemtype.options[ddlItemtype.selectedIndex].value;
if (ddlitem == 0) {
alert("Please Select Country.");
return false;

}
function IsValidEmail(email) {
var expr = /^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$/;
return expr.test(email);
};
function ValidateEmail() {
var email = document.getElementById("txtemail").value;
if (!IsValidEmail(email)) {
alert("Invalid email address.");
}
else {
alert("Valid email address.");
}
}
function lettersOnly(e) {
var k;
document.all ? k = e.keyCode : k = e.which;
return ((k > 64 && k < 91) || (k > 96 && k < 123) || k == 8);
}
function numberOnlyExample() {
if ((event.keyCode < 48) || (event.keyCode > 57))
return false;
}
}
http://www.c-sharpcorner.com/UploadFile/9f0ae2/gridview-edit-delete-and-update-in-Asp-Net/