If you want to go the easy path, you can let people say a certain word in the comments or just a comment, there are bots for doing so.

If you prefer to not have other bots / vultures which are snanning hive for such giveaways, than you could go for a reblog lottery. You can choose how long it runs. There are people running the lottery at post payout or 24h after being posted. I am not sure, but guess there would be bots for this as well.

If you prefer to have engagement, let people answer a certain question / post a picture in the comments /...

About the bots, I am not sure where you could exactly find them, if you don't program 'em yourself. 🙈