Controls Webpage with Knockout: Visible changes not saved

As the regarding web site can not be shared, I can come up with a just set of hints to try out:

If the web site would implement a simple (pure) HTML form to send the POST request, your solution would be fine. But looking at the HTML you shared

<label data-bind="text: ProjectFile().ProposalDpLabel()"></label>

the data-bind is already suggesting that the data is getting collected/sent by a library. (E.g. Knockout is using that attribute). This library might now collect the data somewhere, and it might get triggered by a "click" or a "key" event in JavaScript. The collected information can then be stored in a hidden DOM element as suggested by GCSDC or directly in a JavaScript variable.

What I would suggest now is to find out which JavaScript framework is used on this page by inspecting the HTML source. At some point there should be a

<script src="<fancy js framework>.js"></script>

tag in the HTML, which should give you the name of the framework. (There can actually be multiple tags of this kind, including custom JavaScript files. These tags do not have to be at the beginning of the HTML document, and can be scattered all over it, so you might have to search for script in the HTML document. One of them should be the main framework, which is sending the request. If you are not sure which one it would be, you have to google all of them and find out.)

Then, research how the the POST (maybe Ajax) request is sent in the JavaScript code on this page, with help from the documentation of the Framework. And then, send the request by executing custom JavaScript from VBA on this page; how this could be done is shown in this post.

Alternatively, you could try to trigger a click (or key) event on the form inputs to make the framework believe you actually typed it in; how this could be done is shown in this post, but this might not work in all cases.