Let's be clear that if they aren't using the Forms (JS) API, but merely posting form-encoded data to the forms HTTP endpoint, they aren't really posting via API. They're skipping the API and going straight to the ep. Not that it doesn't work, it just is a level deeper than the Forms API.
Anyway, just as a form might have zero fields in Form Editor but can still accept data for any fields added in the browser (i.e. if you add fields using the Forms API addHiddenFields method), yes, the endpoint can indeed accept data for any fields (using SOAP names) in the form data.
Thank you, Sandy!
Just to be clear on my end, I have no idea which method is actually being used. I just have to supply the form. And I think you are saying that either way - it doesn't matter what fields are on the form. To give a silly example. If the Form were called "ABC Form" and had only these 3 fields:
The entity posting to the form and populates Email Address and a field called "Favorite Monkey" with Howler Monkey (let's assume Favorite Monkey is empty prior to the form fill).
And if I created a Smart List with:
Filled Out Form->Form is: ABC Form
Favorite Monkey is Howler Monkey
The list would show members?
P.S. Please do not construe this as a preference for howler monkeys.
Correct. It doesn't matter which fields are shown in Form Editor. Any fields in your instance can be posted (I was giving the addHiddenFields method as proof that it doesn't matter, since that method adds new fields dynamically on the client side and the server has no idea until it receives the post.)