I'm pretty sure it's only for direct links to the video. fun fact: if you have Discord Nitro you can just upload the video to Discord and then just right click it and then click copy link and then you can use it like that
as i know it doesnt support youtube because of API, also i dont have discord nitro. so i wanna know if theres some web site that i can post a vid and then link it in to the online.txt