Tell it what you know about the person and the moment. You'll get three drafts to pick from, and you can ask for shorter, warmer, or less formal until one sounds like you.
Each rough draft is written near the length you selected, but only the first 25 words are shown at first so the picker stays easy to scan. Reveal any option to read the rest, then polish the one that feels most like you.
Read it once before you hit send. Cross out the lines that don't sound like you and rewrite them in your voice. The point isn't to send this draft — it's to break the blank-page paralysis so the message that's been sitting in your head actually goes out.