You can add a minute to an agenda item using the 'Add minute' option from the meeting agenda item menu — but please note this will be visible to all meeting attendees, not just yourself. e.g.
If you prefer to keep something private, you can use 'Add action' and select your private personal board (which only you can see). The action link from the meeting to your personal board will remain private — others won't see it at all.
Tip: You can reuse the same personal action item across multiple agenda items by selecting 'Link an action' (instead of 'Add action'). This way, one action item serves multiple meeting agenda items.
How to Link Actions in a Meeting Agenda for an Upcoming Meeting