I should have mentioned this in the post but unfortunately there's no way to set beneficiaries on a post through the SteemIt.com site (or any other sites that I am aware of) right now.
However just earlier today @stoodkev released an updated version of the SteemPlus browser extension which will allow you to set beneficiaries on posts you make using the extension.
You can read about it and download it here: https://steemit.com/utopian-io/@steem-plus/steemplus-1-7-share-your-rewards-with-your-friends-beneficiaries-ideal-for-the-steemfest