Dear Timothy,
All changes only in " View Query – Choose what content to load" section. Please don't touch "View Layout – Edit the layout" section this time.
For first you need to modify "Custom field filter": like at my example in attachment:
where url param "thumbnails" this is parameter what will be a key in url.
After this you need press "View/Edit Meta HTML" and add code for search:
<div class="sbox">
<label>Thumbnails</label>
[wpv-control field="thumbnails" url_param="thumbnails"]
</div>
Please notify that url_param must be same as at "Custom field filter" url param.
After saving layout you can see additional input in form and search results after submit.
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.
Regards,
Vlad