Fill in form space with underline

[eteague]

I am creating a fill in the blank form. I have inserted a plain text content control box for those filling out the form electronically but I also want to have an underline in the same position on the from for the person to write on if they are filling the form out by hand. Simple enough - however, when the form is filled out electronically, it inserts the entered text into the underlined space on the form making it longer, how do you make the electronically input text overwrite the manual fill in the blank line with a matching underline format? In other words, I want to have an underlined space that can be filled in electronically or manually.