I've been looking for an answer to this too, but i haven't had any luck. This is what i did. I added the 'name' attribute to all my input fields where i want to add a placeholder, since we are using forms from eloqua this attribute will be set by default; change the content of the attribute with whatever you want to have as placeholder. Example <input type="text" name="firstname" /> changes to <input type="text" name="First Name"/>
Once you have all your attr set, add this call on your js tools (you find this under "Tools")
$('input[type="text"]').each(function() {
var placeholder = $(this).attr('name');
$(this).attr('placeholder', placeholder);
});
this will create all the place holders for your inputs. if you want you can be more specific and doing them individually with this code:
$('input#INPUT-ID').attrr('placeholder', 'Your place holder text');
I hope this helps, good luck!