[SalesForce] SmartCapture form not validating & not passing hidden attributes

We want to enrich existing data about customers using a SmartCapture form. So, what we decided is to send an email to the existing customer audience with a link to a CloudPage with the SmartCapture form related to a new & empty data extension in which we would have additional fields such as FirstName, LastName, etc.. Currently, we only have the email address and the FederationID in the campaign audience. Also, as part of the user experience, we would want to pre-populate the email address field and pass the federationID (i.e. Primary Key) as hidden attribute because it does not make sense for the customers to know what their federationID. The end goal is that the SmartCapture form will populate a brand new data extensions (an empty one), and then we would to cross-reference (SQL) the newly enriched data extension with the original audience to which the email was sent via the FederationID.
However, I’m stuck. Here’s what I’ve done.

• Email

  The email to be sent contains the following code I wrote behind the link to the landing page:

  <p>follow the `%%=RedirectTo(CloudPagesURL(985, 'EmailAddress', EmailAddress, 'federationID', federationID))=%%">link
  

• Cloud Page – SC form settings

  On the Cloud Page, the SmartCapture form, in the settings, it has under
  "HIDDEN ATTRIBUTES", federationID:

  %%=AttributeValue('federationID')=%%
  

  (so we want to pass federationID as hidden attribute on which we would later on cross-reference the resulting data extensions with the original audience)

• Email Address as form field (pre-populated)

  While in one of the SC form fields – EmailAddress – it has AMPscript for prepopulating the field (which it works OK):

  value="%%=AttributeValue(' EmailAddress')=%%"
  

Problem:

The problem is that, although when I click on the link in the test email and see that the EmailAddress field is correctly pre-populated, I cannot hit the “Submit” button. In other words, I cannot validate the form (it’s just stuck, I’m not getting the thank-you message), the target data extension is not populated and it seems to me that MC is not passing the federationID value.
How to make the form validate with federationID being passed under hidden attribute?