How would I translate the phrase, "Please sign in to send the message".
I would think it's something like, "Qing 登录 fa xin" -- is that correct?
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Sign up to join this community请登录来送出信息 or 请登录来发送信息 or 请登录以便发送信息.
If you mean to say "To send the message, please first sign in", then “发送信息前请先登录” may sound more natural and clearer.