+1. I have also searched high and low for the very same thing you are describing. It seems like a big limitation in defineCustomElement to not being able to pass arguments to the underlying CustomElement constructor. In this case attachInternals & static formAssociated properties.

Did you find any solution to your problem?

Sidenote:
Here is an older issue (see latest comment) I found, which also adresses some of the same problems.
#vuejs/rfcs#220

The only way to bypass this is to set a hidden input with the right name and value thru js but it kinda defeats the purpose of creating a custom element in my opinion.

I think we could have a formAssociated option in defineCustomElement, and add that static prop in VueCustomElement

static formAssociated = !!Comp.formAssociated

ElementInternals should be attached if we want to make it associate with form.
ElementInternals can also be used to access Accessibility Object Model and Custom Element State. So we should call attachInternals if supported (Or add another option?)

constructor(initialProps?: Record<string, any>) {
  super(Comp, initialProps, hydrate);
  if (this.attachInternals) this._internals = this.attachInternals();
}

Then, users can utilize _internals if they want
But not sure if we should just add formAssociated as a common component option(for .vue file), I am using this solution for defineCustomElement in tsx only. If that's ok, I can make a PR for this.

For temp solution, check out this, I made some changes based on your demo

I came to the same alternative approach as you @lejunyang. But in my opinion that kind of defeats the purpose of using vue with web components with such a custom solution.

If this is something that won't implement then I think there is an argument that this should be reflected in the docs; https://vuejs.org/guide/extras/web-components. As of today the docs makes it sound "plug and play" when working with web components and vue. Which is to some degree true, but there are lacking functionality.

Any news as to what might happen with this request?

Does 37d2ce5 help at all? I would have assumed not using the Shadow DOM should allow the <form> to pick up the input elements as usual.

I will try but I doubt as the html spec requires it to be a form-associated element using that static property within the class and having the _internals property accessible