I can't find where we activate the game for preload.

Are you using Galaxy or not?

Yes
I use it

Very top of the client, you click library, select Witcher 3, click install, and that's you. Still have to wait another 7 hours to play though, that is when you need to download a day one patch to activate the game.

But me it's a redeem code, i bought it on kinguin

You will see a + sign on the top left side of the client, click on 'redeem code', and that's you.

On the top left you have the search bar and a plus sign + on the right. Click there. Select Redeem code

He told me invalid code :(
I bought it 1 hour ago wtf ?

Are you sure that site / store is legit ? :p

It's like G2a and i bought GTA V on it
Problem solved