This is how to enable/allow the use of iframe tags within the Tiny MCE (TinyMCE) editor.

Simply within the tinymcs-init.js file put the following line of code:

extended_valid_elements : "iframesrc|width|height|name|id|class|align|style|frameborder|border|allowtransparency",

The init file can be found in the root of the tinymce directory.