Did you already added it under Events > Settings > Formatting > Calendar > iCal Feed Settings > iCal Description
e.g. the result should be something like
BEGIN:VEVENT
UID:[email protected]
DTSTART;TZID=Europe/Madrid;VALUE=DATE:20211227
DTEND;TZID=Europe/Madrid;VALUE=DATE:20211228
DTSTAMP:20210324T101852Z
URL:https://your-site.com/event/zoom-meeting/
SUMMARY:Zoom Meeting
DESCRIPTION:27 Dec 2021 @ All Day --- https://zoom.us/j/96154620154?pwd=VXFRTjh0ank0KzFIYkcrVENCWkdLdz09
CATEGORIES:apple,Demo 1,Demo 2
END:VEVENT
You can also try to modify the template file under events-manager/templates/templates/ical.php
to use templates: https://wp-events-plugin.com/documentation/using-template-files/
eg. wp-content/themes/Your Theme/plugins/events-manager/templates/ical.php