When in an Estimate, to create a Follow-Up Override, you will scroll to the bottom of the page and select "New Event".
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Once "New Event" has been selected a small window will pop-up giving you the option to leave notes, as well as schedule a "Follow-Up Override".

Whichever date is selected from the calendar drop down will be the date the Follow-Up will appear in your "Follow-Ups" Tab at the top of your screen.