Makking domain auto-add www

I’m using my web address, which I registered on Dreamhost, with Tumblr. I used Tumblr’s instructions and added


and works just fine. But when I leave off the www, it errors out. I know when the site is fully hosted there’s a box to check to add the www; how can I do this with just the DNS hosting?

See this article. It appears you may need a second CNAME record.
although I don’t actually understand the VALUE for the second record as described there.

I think that article is out of date. The second CNAME record is actually what USED to be used by Tumblr, before it changed to a few months ago. I’m sure there’s some simple thing that I’m missing. Just can’t figure out what it could be.