Well its redundant to call a JavaScript with a asp button when you can always just use the on click method with a normal html button.
<button onclick="onButtonClick()">Click me</button>
However here is how you can do it with the following ways
Response.write("<script type=""text/javascript"">onButtonClick();</script>")
// OR
ClientScript.RegisterStartupScript(GetType(), "Javascript", "javascript:onButtonClick(); ", true);
// OR
string name = "onButtonClick()";
ScriptManager.RegisterClientScriptBlock(this, typeof(string), "Key", name, true);
// OR
ScriptManager.RegisterStartupScript(this, GetType(), name, false);