Select the TextField and set font style to Underline.. It will place an underline to Caption also, if you have it. If you want to remove underline for Caption just select Caption text only and then uncheck the Underline for Font Style..
This is the only way I know of adding lines to multiline text field.
There is another one "Comb" -ination of characters which will work only with single line text field.
Hope this helps..
If I understand your requirement correctly..
Some users can fill the form online and some users will take print of the form and then write their response.
If that is the case, I can think of other option..
Add a table to your form below the Text Field and add one column with only one row. Set the Bottom border only for the Row. Check the checkbox for Row "Repeat Row for each Data Item". And set the presence property of Table to Hidden. Make the form as Dynamic XML Form. To make this work, I need to set the Page1 as Flowed Subform..
Check the atatched sample file..
If the user enter the value in the TextField, the underline will be placed automatically since I set the Font style to Underline.
If the user prints the form, then I am hiding the Text Field and displaying the Table Row and setting the rows to 30. (check the code in prePrint and postPrint events).
To test the sample, open the file in Reader/ Acrobat and then click on Print.
Hope this gives an idea..