I know this is an old topic and you’ve either solved it or moved on by now, but for anyone that has the same question, this isn’t something you can do in ACF. This would take 2 separate fields, a checkbox and a text field. You might be able to code an acf/save_post filter to take values entered into the text field and update the checkbox field if your goal is to allow values to be added to the checkbox field.
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