There are a couple of ways to do it: you can do a bitcoin giveaway in the Games and Rounds section, or you can do an altcoin bounty and post it in the Bounties (Altcoins) section, but you cannot do an altcoin giveaway anywhere on the forum. For that you will have to make an off-site giveaway and post the link on the forum. It is important to note that for altcoin bounty, participants must complete some task (for example: follow/like/share on social networks) but you cannot reward them solely for replying to your thread.