The form builder only knows which variables are being used if you add them to your form by using the built in elements. What you'll need to do is keep the element holding the html hidden with some css.
#myhtml{
display:none;
}
All variables have to added to the form through the form builder app for submission. You can have them hidden as long as you don't use the built in conditionals and add a html element with the proper css to keep them hidden. You need to do is either use Javascript or JQuery to pass the values to the input in the form.
It's easy to overlook something you're not looking for.
This is a site I built for my work.(RSD)
http://esmansgreenhouse.com
This is a site I built for use in my job.(HTML Editor)
https://pestlogbook.com
This is my personal site used for testing and as an easy way to share photos.(RLM imported to RSD)
https://ericrohloff.com