My first attempt failed. They have a URL scheme documented but the macOS app does not seem to support it. All the URL does is open the app, it doesn’t add a todo.
This is the attempt that didn't work...
#popclip
name: Twos
url: twosnotes://todo?text=foo
(The above block is an extension snippet — select it then click “Install Extension” in PopClip.)
Next approach will be to see if the have a api for the backend … It seems they do but it is unofficial. The way to get the token is very complicated, involving installing a browser extension and sniffing the dource using browser dev tools!