When saving after attaching the video in a HTML field [Odoo V10], the video link or even the video disappears. So unable to access it after saving.

For Example:

In the sale.order module I added one HTML Field in new Page. I added a HTML Field in python and I given below

demo_video = fields.Html(string='Demo Video', help="Demo Video")

In the HTML Field I am adding one Video Attachment from the Youtube Link with the given followed syntax,

//www.youtube.com/embed/KxZAdEGpYAw

From the video preview it showing the video in html field after saving the record the video will disappears in the Field.
Is there any way to make it appear?