How to consistently make a newline inside a message, in the different messaging apps?

🟢 Alt+Enter = 83% 🟡 Shift+Enter = 67% 🔴 Fn+Enter = 33%
Adium ✅ ❌ ✅
Facebook Comments ✅ ✅ ✅
Facebook Messages ❌ ✅ ❌
iMessage ✅ ❌ ❌
Skype ✅ ✅ ❌
Slack ✅ ✅ ❌