Can you leverage Custom Program Member fields on a form embedded on a 3rd party landing page?

Hello!

I am trying to confirm if you can leverage Custom Program Member fields in a Marketo form that is embedded on a non-Marketo landing page.

In my early testing it appears that you can put the fields on the form, the form submits and you can see the data in the form submission activity, but the Custom Program Member fields do not get updated. But when I do the same on a Marketo-hosted page, it all works as intended.

Is this how this is expected to function?

Thanks!