There are two possible ways to do this:
1. Use a 1x1 table and remove the borders except the bottom border. This will allow people to type in the empty table without shifting text around. The limitation is that the 1x1 table must be on its own line.
2. The preferred option is to use the Developer ribbon (enable Word Options - Popular - Enable Developer...). Then under the controls, insert a text box. You can then edit the font of the text box to add and underline. You can find more information about this method on Microsoft's web site here