Hello thaigerweb,
We mean to say that, if you want to add the video directly on the content section, you can add the iframe on the content. But, if you want to use the featured video, you can use it as well. Please have a look on this post to know how to use the featured video. In the above post, you missed to add the featured image and missed to replace the featured image on single post with video. Right?
Can you please check all the options once.
Let us know, if you have any confusion.