I don't think there's any very good way to do that. You certainly can't do it by applying bold to underlined text, because Word accomplishes that by just thickening all the lines in the text including the underlines.
There are two variations of the EQ field that might come close. One uses the box-drawing switch with an option to draw only the bottom of the box, and the other uses the overstrike switch to place underscore characters under the text. In both cases, you need
to display the field code and apply bold format to the text but not the rest of the field.
For the box-drawing variation, create a field code like this, using Ctrl+F9 to make the field braces and replacing 'this is bold' with your text:
{ EQ \X\bo(this is bold)}
Apply bold format to only the text between the parentheses. Update the field. This draws an underline that's even thinner than the ordinary underline in non-bold text, and it's a couple of pixels lower than an ordinary underline.
For the overstrike variation, make a field like this:
{ EQ \O(this is bold,_________)}
Again, apply bold format to only the text, from the letter after the left parenthesis up to but not including the comma. After the comma you need some number of underscore characters -- but unless you're using a monospaced font, the number of underscores won't
be the same as the number of letters in the text, and you'll have to experiment to get the right line length. Also, there will sometimes be a one-pixel gap between the end of one underscore and the start of the next.
In both cases, you need to delete the space character between the closing parenthesis and the closing field brace, or else Word will make it look like there are two spaces between the field and the next word.
In all, it's not a good situation. It may be better just to live with bold underlines.
