How to add note about a lead?

Contact Card is the place where you can edit all contact information, and it's accessible from two pages. Primary access is from Global Data → Any of the sections, by clicking on the card in the Contact column.
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but alternatively, you can access it from AI Scheduler → both Upcoming and Past tabs as well. Just locate the contact, expand the call details and click on the email to access the contact card.
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Once the Contact Card is opened, it will land on Journey tab, by default. To add a note, switch to the "Notes" tab and start adding information in the text box.