Hey Tuanphan
I'm having a similar issue on one of my client sites: https://snail-gopher-y6jg.squarespace.com/ (pwd: lush)
I've tried the code you suggested above, modified the id and played around with the measurements, but I can't get it to work! Would you mind please having a look? At the moment I added in a blank section above the bg video on the home page so that it didn't get cropped by the menu, and it works on desktop, but on mobile I get the black bars top and bottom.
There's a bg video on the videography page as well: https://snail-gopher-y6jg.squarespace.com/videography
Thank you!
Rae