You need to do the following:
Go to Extension Settings
Go to Engagement section
Turn on Enable Airtable comments on child forms toggle.
The Comments button is available in the lower right part of the child form. You can view or add comments from here.
This feature also works with the form. When the login feature is enabled, the first login field will show up as a prefix of the comment. But when you are editing a record using a form, the primary field of the table that your form is editing will show up as the prefix.