Hi there @emeraldcolour,
There is no built-in functionality to make that checkbox required, since that would defeat the purpose of it being an “opt-in”. If it were required, then they would not be “opting in” to receiving marketing emails.
If you are certain that in your region it would be legal to have it pre-checked, then that could be possible with some custom code.
If you need help with custom code, we recommend reaching out to someone from Codeable: https://codeable.io
That said, we do recommend allowing customers to opt-in instead, and keeping the double opt-in feature enabled as well (required if sending via MailPoet Sending Service):
https://kb.mailpoet.com/article/128-signup-confirmation-double-opt-in-feature