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There's always the tried and true method: Type your message in notepad, then copy and paste it into wherever when ready.

Yes, "Facebook™ Chat Privacy" Chrome extension:

Anyone else get an error when trying to add this extension?

>Package is invalid. Details: 'Could not load background script 'tracksy/tracksy.js'.'.

There's one called Facebook Unseen that works well. It's a Chrome extension that gives you a toggle.