Although the title of the Shopify cart memo can be changed by "Edit Default Theme Content", it is not possible to add any comments. Here is how to set up the theme "Dawn" so that you can post supplementary information for the cart memo that can be edited with custom Liquid.
Example of setting to add a custom Liquid input field to the right of the cart memo field (below on mobile screen) in the theme "Dawn" so that comments can be freely inserted
Setting Step 1: Add the schema to "settings_schema.json" and the label to "ja.schema.json"
Setup Step 2: Add code to display custom Liquid in "main-cart-footer.liquid"
Setup Step 3: Go to "Theme Settings" > "Cart" and set the content you want to display in the Liquid code editing box.
