Hi Scott - We have a script for adding hint text to your form, since you can’t edit the form fields directly. You can find that, along with instructions for personalizing it for your page here.
One thing that I should note though, is that hint text often doesn’t increase opt-ins. Some tests have found that fields with text in them are less noticeable and if you’re using inline text without any traditional labels, it can strain a visitor’s short term memory (if they click on field and the text disappears and then are drawn somewhere else on the page). That said, I definitely have talked to people who have seen it work for them as well.
With that in mind, I’d definitely recommend testing a variation with the hint text and one without. I’d love it if you could share your results as well!