Write Google Form Dynamic Content Before Submitting

I've been trying to search for this answer and I feel like I keep bumping up against it but can't break through.

I'm building a Google Form for a non-profit. Part of it involves filling out multiple fields where you can agree to a donation to separate funds. I would like to dynamically total those donations fields and display the total for the end user before they submit the form.

I can't seem to find a way in the AppScript Form API to trigger onChange of a field and get at those values (or how to write them back to the screen).

Interestingly, there is a 3rd party plugin that I can pay for that does it. I figured if someone else is able to do it with 3rd party code, why can't I?

I just can't figure out how they're doing it.

I've explored the App Script API and dug through a lot of previous questions. Nothing quite seems to hit this one.

The Google Forms API only handles the creation of forms or what happens after submitting a form. You can’t make dynamic changes like live calculations, or using information entered on previous questions to feed new inputs and display them in the user’s view.

I played around with Formfacade, and it seemed to me that it operates from a separate environment in HTML; my guess is that the customized fields are mapped to the questions of the target form but that overall what you see is their visual environment, so it is not necessarily using the Forms API but a custom library for their service. Nevertheless, having the ability to work with dynamic answers in Google Forms would be a nice feature to have. If you have the time, you can open a Feature request

References:

• How to add a simple calculated field to a google form?
• Google Forms triggers
• Google Forms API REST