I didn’t think that I had to spend quite some time to do this simple task, but for a Joomla newbie like me, it gave me some difficulties. 

• You should install a Article editor called JCE Editor. You can download the module from this link.
• After installation finishes, you have to open Component > JCE Editor > Editor Profiles > select Default > Plugin Parameters > select Media Support. Then, make sure to turn on Allow iFrame 
• Make sure  in System > Global configuration > Text Filter >Super Users, No Filtering is selected.
• Make sure select JCE Editor as your default Article Editor. This is located in System > Global Configuration >Default Editor.
• After all of those above configuration is set, you can paste the iframe code to Article editor (remeber to toggle source code mode in the editor). 

Most of those steps are covered in below video, good luck: