Need to underscore text with JS for PDF form

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Hi All, I need to have text automatically underline as the user types into a text area. I would like to use a javascript to do this, and it seems that it can be done using the annot object and underline, but I am too much of a novice to know how to write the script, any assistance very much appreciated! I am using Acrobat Pro 8.0

Thanks, Dawn