Form with MultiSelect editor does not submit selected values in the MultiSelect.
Add support for hidden fields. There could be an alternative way of adding them to the Items collection, e.g.
Consider the cases when the hidden field is bound to a model field and when it isn't bound to the model. The latter could be a scenario involving antiForgeryTokens:
<input type="hidden" name="__RequestVerificationToken" value="token" />
In both cases you should be able to set the value of the hidden field programmatically and submit it with the form.