adding video to header?

Hi, is it possible to have a video in header? I followed this thread https://make.www.ads-software.com/core/2016/11/26/video-headers-in-4-7/

and added theme support from the first step but i have no idea where to add the_custom_header_markup()

Now I have option to upload a video or add youtube link but video is not showing in header, old header photo is displayed instead.