WORKAROUND
Method 1: Use the Line Tool
Use the Line tool on the Drawing toolbar to draw lines under the text to
simulate underlining.
TIP: Hold down the SHIFT key while drawing lines to keep the line
horizontal.
Method 2: Use an Equation Field to Create an Underscore Character
Create an equation field using the overstrike function to underscore words
or phrases that contain superscript and subscript characters, similar to
the following
Syntax: {eq \o(<Text>,<Underscore characters>)}
as in the following example:
The braces for the equation field are created by pressing CTRL+F9. Type
the
text between the braces.
In the syntax above, <Text> is the string containing all the text to be
underlined (normal as well as superscript and subscript text), and
<Underscore characters> are the actual characters to underscore the "Text"
characters.
NOTE: The underscore character may not match the one used by the underline
character format command. You may need to experiment with different
underscore characters (different fonts, point sizes, and so forth) to
achieve a continuous underscore effect.