I couldn't find a quick way to do it. But then again, I would be recommending you to use a Content Control instead of the seriously out of date legacy fields. A content control will accept formatting in the Placeholder text. Line breaks in the placeholder text are also possible depending on the type of CC.

If the CC already contains content, the user won't see the CC's placeholder text so it isn't a perfect solution unless you are happy for them to clear the content which allows the placeholder text to reappear.

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Andrew Lockton
Chrysalis Design, Melbourne Australia