How do I get an "Ask me anything" box on Tumblr?

The above answer is correct, but you may need to activate it first.

1. Go to your settings (cogwheel at the top of your dashboard)

2. Click your blog's title in the sidebar on the left.

3. Make sure the 'Let people ask questions' box is ticked. Tick the anonymous box too if you want to allow senders to be anonymous.

4. Save and close.

Once you've activated it, you can add a link to '/ask', and you're good to go. How you add the link would depend on which theme you're using and where you'd want the link to be.

For the "ask" hyperlink, that you may go to customise, then community. Then make sure the "Let people ask questions" choice is checked off. You can also alternate the page title, permit humans to ask anonymous questions, and many others. There.

everyone has one. just add "ask" after your own url
eg. http://xxxx.tumblr.com/ask
and there you go:)