What is with Special Keys or codes for InGame Items etc. ?

No. Unless you giveaway Payday 2, you can't do such a giveaway.

If there's a app id for it, then yes. Usually at least. Try to find it as a dlc

No. you are not supposed to misrepresent the game / DLC given away.

If the Mask is in the giveaway list you can go ahead and create the giveaway for this DLC.

However do not create a giveaway for the DLC disguised as the full game.

(Would not make sense anyway. A user who already owns Payday 2 can't enter the giveaway for the DLC and the winner can't activate the DLC without owning Payday 2).

If your code goes in the "Activate a Product on Steam..." window, and you create a giveaway for the actual thing you have, then you should be ok.

However, if you have a code for something, but you can't find that exact thing in the giveaway list, you can't create the giveaway for something else and tell people in the description what your key is really for...

Check the Steamgifts Guidelines if you're curious about any other details. Here are some common mistakes new people make:

All giveaways should be for Steam redeemable gifts or Steam redeemable keys.

Beta keys, guest passes, coupons, and free games should not be given away.

Giveaways should accurately reflect the game being given away. If the game is not available in the giveaway list, please contact support to have the game added, instead of posting the giveaway under an incorrect name.

So also Accessoire or Skins for Games are not available for giveaways ?
Why can´t they add this ? A Skin or as example the Mask for PayDay is independent from a DLC

• Edit ah okay but Accessoire or Skins are excluded / not included in the guidelines/FAQ becasue they are redeeemable Keys via Steam you activate them via "Activate Product" dialoque so i do not understan this

Only stuff with a store appID can be given away. If you have something else, just give it away without a raffle here.