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: