Use the below code to call a javascript function from cs
ScriptManager.RegisterStartupScript(this, this.GetType(), "Name", "postform_open()", true);
User the below code to call to call an javascript from html
<head>
<script>
function postform_open()
{
}
</script>
</head>
<body>
<a href="" onclick="postform_open();">Send</a>
</body>