What the first second has to do
On TikTok the viewer decides with their thumb before you have said anything, so the first second has to show something worth watching. For most products the strongest opening is the product doing its job: a hand, an action, a visible change. A logo, a slow pan across a room or someone saying “Hey guys” gives them nothing to stay for.
Write the first second as a picture, not a line. “A pilled sweater sleeve, the fabric shaver touches it, the bobbles come off in one stroke” is a brief. “Hook the viewer with an exciting opening” is not. If you can’t describe the first frame in a sentence, the hook isn’t decided yet.