Adding submit button inside a fieldset

I'm trying to insert the submit button inside a <fieldset> along with other form fields. I've managed to create a fieldset but there's no way to include the submit button in it.

If there was some way to include the submit button using the forms editor that would be great.

Here's a link of the page: http://www.geckoroom.com/presupuesto-web-a-medida

Thanks for your consideration,