Forms are not getting pre-filled (new record is being created, existing record is not updated)

I’m having a hard time getting forms to be prefilled after sending a double optin email.
I’ve got a fresh Mautic 6.0.3 installation in which I’m trying to get the double optin mail working.

Main goal
When a client receives his double optin email I want to have a form embedded asking to give us more information. This information needs to be stored within his own profile (not a new record).

  • I’ve created an email
  • I’ve created an (optin) form
  • When a record is created a custom field ‘uid’ is automatically filled with an unique ID
  • I’ve created a campaign
  • The campaign sents the email
  • The email has a link to a page on my website (thankyou.php?uid={contactfield=uid})
  • The page on my website (thankyou.php) has the mtc.js (tracking) script included
  • The page on my website embeds the form with the given javascript code

So far everything works, but: the form will not prefill the clients information. When the form is submitted a new record is being created and I would like that the clients record gets updated with the values given.

What to do?
I cannot get the form to be prefilled when the page is visited after clicking on a link in the email.

Can anybody assist and help me out?