As you need to control the date/time at which the content becomes visible, I would rather use 2 landing pages:
- The first one would display on a URL until the morning of the day.
- On that day, I would switch the URL to another LP with all the content.
And I would communicate the URL through multiple reminder emails.
It would be easier if this idea was implemented: Automatic Landing Page URL Switch
and keep the URL hidden using noindex, nofollow to ensure random people can't find it.
But if you really want a portal or something, you should speak with your web team and not host it on marketo.
Maybe you were thinking of the Known Lead HTML feature, not ProgPro.
Easier than trying to exploit KL HTML would be to include a hidden field indicating they've signed up for that particular event. Then in the form's whenReady listener, you can check if that field is filled in. If it is, swap out the whole of the form HTML for your button, or if the on-demand date hasn't passed, give them a third state "On-demand sessions are still being prepared."
But like Josh said, Marketo forms aren't meant to handle anything we'd call a "login." Anyone can impersonate a lead just by entering their email address, so you're really talking about zero-factor security.