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If your Checkbox field's field identifier is nine then you can put this on your Exact Form:
Replace the nine above with the field identifier of your Checkbox field.
To learn how to determine the field identifier of your field please read the following:
Just copy and paste the line above that begins and ends with a tilde (~) character into your Exact Form. Then replace the nine with the field identifier of your field. Then replace the word
with the Webdings character that is a checkmark by typing a lowercase 'a' on your keyboard. Leave the double quotes that surround the word checkmark alone. Then set the entire line's font, including both tildes, to Webdings. This will make it unreadable, or even more unreadable, but it will work.