Calling inline Javascript from codebehind on pageload

Calling inline Javascript from codebehind on pageload

I have a Javascript block on my asp.net page:
<script type="text/javascript">
Sys.debug = true;
var popup;
Sys.require(Sys.components.popup, function () {
popup = Sys.create.popup("#popup", {
parentElementID: "target",
});
});
</script>
How do I call this from C# code behind?
ClientScript.RegisterStartupScript(GetType(), "key", ???, true);