To embed a video in the Detailed text area:

(1) Click on the Add/Edit Details link to open the Details editor.

(2) Switch to the Html editor, using the < > button (in the bottom left hand corner of the Details editor – below the text entry  box).

(3) Paste the video embed code into the text entry box.

(4) Change the width of the video to a value below 365 pixels – usually using a fraction like 4/5, 3/4, 2/3 to accomplish this – while keeping the height and width ratio the same (rounding up/down if appropriate).

(5) If necessary (though generally it isn't), iterate on (3) until you are happy with the result. 

(Note: that sometimes the video embed code will include the height and width values twice, and, if so, it is advisable to adjust the values for both instances).