2019, Washington D.C., USA. Looks like they require online applications now too, though, so OP probably just needs to ask what specific option he's supposed to choose on the website at his location.
My family piggy backs on my voluntary visa, and we just had to submit everything for each person on the Non O volunteer list except for the volunteer specific documents (application, passport copies, etc.), plus you'll need proof of relationship (marriage certificate and birth certificate copies). I didn't do an online application though, just the printed application provided by my local Thai embassy in my country, and I wrote Non O dependent for the reason.