Add watermark icons

Hi all,

I was wondering if it is possible to use the watermark option with icons.
For instance, [email* your-email watermark “e-mail *”] shows “e-mail *” in light gray, but I would like to show also a light grey envelope from awesome icons (http://fortawesome.github.io/Font-Awesome/icon/envelope/).
I tried pasting the code <i class=”fa fa-envelope”></i> in different ways but nothing worked.

Any idea? Thanks!

https://wordpress.org/plugins/contact-form-7/