Someone asked me if I knew how to do this the other day, and I had helped someone do it before, but apparently MailChimp support said to this individual that it wasn’t possible/couldn’t be done/etc. I found that a little odd, because it’s a super easy thing to do. It’s true that the tools they provide won’t let you do it, but with just a few modifications to even the stock code the MailChimp form generator creates you can easily do this.
First thing you’ll want to do is generate your MailChimp Opt-In code as you usually would (be sure to check ALL fields, not just required fields when prompted). Once you have your Opt-In code paste it somewhere, such as a text editing window, where you can easily view it and edit the contents in plain text. Shortly below the