You'll need a bit of code to handle this:

- define a CSS class like this in the project CSS code:

.my_underline {
    border-bottom:1px solid black;
    padding-bottom:3px;
}

- then open the code of your text box and apply the class to the span of text you want to underline, e.g.:

<p>Lorem ipsum dolor sit amet, <span class="my_underline">consectetur</span> adipiscing elit. Phasellus semper est vitae feugiat elementum. Duis egestas sem sed tellus commodo, eu rhoncus ipsum dapibus. Sed ac metus nunc.</p>