There is no underline parameter in the default text inspector unfortunately. This is a function of the specific font you are using. The Line Reveal title (standard in FCPX) provides an underline with limited abilities. The line remains the color of the text with no option to separate the two. You can adjust the line length, width and opacity.
You could open a copy of the Line Reveal title in Motion and program an underline with specific parameter options, or you can look for a variety of title plugins that come with optional underlined text - Or just find a font that has an underline option and that should work with any title.
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