it is apparently possible to have an URI with iCal inside the HTML

<a href="data:text/calendar;base64,XXXX">📅</a>

With Base64 encoded iCal thingy:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//hacksw/handcal//NONSGML v1.0//EN
BEGIN:VEVENT
UID:20231024T123432Z-541111@example.com
DTSTAMP:20231024T123432Z
DTSTART;VALUE=DATE:20231028
SUMMARY:update your VPN config file
END:VEVENT

Could be neat and relatively easy to implement. Should of course also include a link to the portal.

Also need to consider CSP whether this is not prevented by the Apache config.