The best alternatives to Secret for iMessage are unSend.it, CyberDust for Android, and Cyber Dust. If these 3 options don't work for you, we've listed a few more alternatives below.
When you see a knoc.me like this: knoc.me/1234567 You can either the URL into any browser or enter the 1234567 into the app to start chat. Just display your own knoc.me on your front door, on the dashboard, or anywhere visitors can physically see.