Dawn Theme - Custom HTML for Video Autoplay

Thank you for providing a solution, however, I’m not too sure I understand this. Do I add that code to the sections > custom-liquid.liquid part? I cut and pasted the code, updated the video url, but nothing happened. Do I need to update elsewhere? Would you be able to provide a detailed step by step to someone who’s not familiar with coding, or maybe point me in the right direction? I appreciate any help you can provide.