leaky-paywall-toc-field.php

<?php

// add toc field to registration form
add_action('leaky_paywall_after_password_registration_field', 'zeen_custom_tos_registration_fields');
function zeen_custom_tos_registration_fields()
{
?>

    <div class="form-row">
        <input style="display: inline;" type="checkbox" required id="tos" name="tos"> <label for="tos" style="display: inline;">I understand I will be automatically billed on my renewal date unless I cancel, and I agree to all <a target="_blank" href="<?php echo home_url('subscription-terms'); ?>">terms and conditions</a>.</label>
    </div>
<?php

}

add_filter('leaky_paywall_account_setup_validation', 'zeen_validate_tos', 20, 2);

function zeen_validate_tos($errors, $fields)
{

    if (!isset($fields['tos'])) {
        $errors['tos'] = array(
            'message' => __('You must agree to the terms and conditions to continue.', 'leaky-paywall'),
        );
    }

    return $errors;
}