Here I have created sample that will help you out.
i have referred below article
HTML
<div>
<script type="text/javascript">
function GotoDropDwon(evt) {
var keyCode;
if (evt.keyCode > 0) {
keyCode = evt.keyCode;
}
else if (typeof (evt.charCode) != "undefined") {
keyCode = evt.charCode;
}
if (event.keyCode == 13) {
var dropdown = document.getElementById('ddlGender');
dropdown[0].focus();
}
return false;
}
</script>
Enter Age :
<asp:TextBox ID="txtAge" runat="server" onkeypress="GotoDropDwon(event)" />
Select Gender:
<asp:DropDownList ID="ddlGender" runat="server">
<asp:ListItem Text="Male" />
<asp:ListItem Text="Female" />
</asp:DropDownList>
<br />
</div>
I hope this will help you out.