Just use the video number, which you can find at the end of the URL.

For example, let's say you have this Tiktok, of the High Roller in Vegas:

https://www.tiktok.com/@austincastrotv/video/7019794562675739910

You would take the number at the end and do this:

[tiktok]7019794562675739910[/tiktok]

And it would show this:

https://www.tiktok.com/embed/7019794562675739910