I am using a vanilla implementation of the Umbraco registration snippet (which uses UmbRegisterController). I want to include custom input fields in the form, but these values simply need to be passed to the page the form is submitted to. E.g. I have Umbraco Member fields, and also vanilla html fields:

// Umbraco Member Registration form field:
@Html.TextBoxFor(m => registerModel.Email)
// Plain old HTML form field:
<input type='text' name='someCustomField'>

And I want to be able to access someCustomField on the next/submission page like this:

if (TempData["FormSuccess"] != null) {
    <span>You entered:</span>
    <span>@Request.Form["someCustomField"]</span>
}

But the Request object has no Form values. Are these in TempData? How can I get the value for someCustomField ?