Call JavaScript function (in aspx) on aspx.cs using a button

I have you this but this is not working 

protected void button_Click(object sender , EventArgs e)
{
        string jsMethodName= = "NewPage()";
        ScriptManager.RegisterClientScriptBlock(this, typeof(string), "uniqueKey", jsMethodName, true);

      //or
      //ScriptManager.RegisterStartupScript(this, GetType(), "NewPage()", false); 
}

Actually I want to open popup modal

Kindly Help 

Thanks In Advance