Slack channel can be overridden to post to another channel/private group or send direct messages. Need to make these database changes:

INSERT INTO settings VALUES ('integration_slack_webhook_channel', '');
ALTER TABLE project_integrations ADD COLUMN slack_webhook_channel text;

app/Template/project/integrations.php

@@ -85,6 +85,8 @@
 
         <?= $this->form->label(t('Webhook URL'), 'slack_webhook_url') ?>
         <?= $this->form->text('slack_webhook_url', $values, $errors) ?>
+        <?= $this->form->label(t('Channel/Group/User (Optional)'), 'slack_webhook_channel') ?>
+        <?= $this->form->text('slack_webhook_channel', $values, $errors) ?>
 
         <p class="form-help"><a href="http://kanboard.net/documentation/slack" target="_blank"><?= t('Help on Slack integration') ?></a></p>