How to make a Checkbox custom-field NOT CHECKED by default always?

hello,

i'm using a custom checkbox field to make the user approve the TOS instead of the built in field (since the built in field use Joomla's Content and i use K2). i'm using the same field for all forms (5 forms currently).

i want this field to always be Unchecked when the form is loading so the user will have to check it on each purchase.
the problem is that after 1 purchase on the site, all forms are loading with this field checked, probably beacause it has been checked on the first purchase.

so i want to make it unchecked by default and prevent it from being checked automaticlly.

i guess there is a way to do it via the "Extra Attributes" field of this field's Display Settings, but i have no idea how.
can you please tell me if that's possible and how to set it up?

thank you very much

Actually, this would require small modification to PHP code. So please submit a support ticket sending us super admin account of the site and link to the form, we are going to check and correct it

Regards,

Tuan