run button's click on page load

run button's click on page load

i have a input like this "
<input type='button' name='osx' value='Demo' class='osx demo'
runat="server" />
that when i click on this , it runs a jQuery plugin. now i want to call
this input's click event on my page load, in fact i want to run it's
plugin at page load, so i use this code :
<script>
$("document").ready(function () {
window.getElementById("osx").click();
});
</script>
but when i run my page , i get this error :
Line: 16 Error: Object doesn't support property or method 'getElementById'
can enyone help me ,please?