How to publish videos on the forum

Do you mean your BiliBili videos don't embed?

Links like this https://b23.tv/4NWMwGl don't embed, you need to open it in a new tab and copy the whole url before the ?
So this link work www.bilibili.com/video/BV1Wb4y1H71f

Where can I find the option?

You go to the URL on your computer/laptop in the URL bar

But the computer I use is a desktop computer.