Thank you for your answer.
The module is onepagecheckoutps, just in case.
I'm guessing this is the part of the code to edit, but I'm not sure:
//add checkbox guest checkout
if (Configuration::get('PS_GUEST_CHECKOUT_ENABLED')) {
$new_field = new FieldControl();
$new_field->name = 'checkbox_create_account_guest';
$new_field->id_control = 'checkbox_create_account_guest';
$new_field->name_control = 'checkbox_create_account_guest';
$new_field->object = 'customer';
$new_field->description = $this->l('Create an account and enjoy the benefits of a registered customer.');
$new_field->type = 'isBool';
$new_field->size = '0';
$new_field->type_control = 'checkbox';
$new_field->default_value = (!$this->context->customer->isLogged()
&& !$this->context->customer->isGuest()
) ? (int) $this->config_vars['OPC_PRESEL_CREATE_ACCOUNT'] : '1';
$new_field->required = false;
$new_field->is_custom = false;
$new_field->active = true;
$opc_fields[$new_field->object.'_'.$new_field->name] = $new_field;
}