There is an even simpler way than the previous suggested code. When you initialize tinymce, it is possible to set configuration parameters. Just make sure you set the AUTOFOCUS option.
var editorOptions = { script_url: "/scripts/tinymce/tiny_mce.js", theme: "advanced", entities: "", theme_advanced_toolbar_location: "top", theme_advanced_toolbar_align: "left", theme_advanced_statusbar_location: "bottom", theme_advanced_resizing: true, content_css: "/styles/site.css", template_external_list_url: "lists/template_list.js", external_link_list_url: "lists/link_list.js", external_image_list_url: "lists/image_list.js", media_external_list_url: "lists/media_list.js", theme_advanced_buttons1: "bold,italic,underline,strikethrough,|,bullist,numlist,|,undo,redo,|,link,unlink,anchor,image,help", theme_advanced_buttons2: "", theme_advanced_buttons3: "", theme_advanced_buttons4: "", width: "640", height: "220", auto_focus: "body2" };
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