you are preventing postback of button click
use this
<asp:LinkButton ID="linkbutton" runat="server" onclick="linkbutton_Click" OnClientClick="return generateRandom();" >Click me!</asp:LinkButton>
and javascript function
function generateRandom()
{
$('#randomText').val(random());
alert('Reference Code: ' + $('#randomText').val());
return true;
}
and on server side get value of textbox.