AFAIK, this isn't possible to do in a single text clip. Multiple clips, you could use underscore and stack the two of them (or use the shape generator to make a thin rectangle) or open in Motion and add one or more lines.
How you managed to do it in a single text clip is a mystery.
If you do it again, hopefully you'll share the solution by posting.
You can use this:
It will only handle a single line, but with baseline and matching text, it can be inserted into (layered with) another Title (if carefully lined up.)
Here's an example:
The underline is linked to the position of the text (changing the alignment to center or right will offset the underline... no big deal - see below.) Changing the position of the text in the canvas will not affect the relative position of the link with respect to the text.
When you first apply the title, the text 'Simple Text Underline' will be underlined.
Use the Offset to slide the beginning of the line to the position you require; use the Length to scale it back to fit the word.
Vertical Offset will drop the underline (or raise it for a Strike Through) -- helpful if the font metrics are non-standard.
[Click and drag on the number values instead of the sliders -- the sliders really don't span that much distance.]
Color (underline only - color the text in the text inspector): this is not linked to the text color to allow different colors like Red for emphasis.
Width will thicken the underline (to match styles [bold etc.])
Use Drop Shadow *will* apply the text drop shadow characteristics (from the text inspector) to the underline. Make sure that the Drop Shadow in the Text inspector is left on, and turn it On or Off in the Title inspector (with the Use Drop Shadow option).
I tested this with the Custom Title and I see the same underline that you see. My theory is that this underline will not show up in your finished movie. In fact, the underline does not show up unless you are in the Title Inspector.
1) With the custom title you get a control (a little circle). You can drag this circle to position the custom title anywhere you want to. The little circle is on the baseline of the title.
2) You also get a Baseline parameter. In the neutral position, the text sits right on the baseline, but by moving the slider, you can have the text spaced above or below the baseline.
So you are seeing the baseline, but the baseline only shows when you are in the Title Inspector. If you preview your timeline, the underline will not be there.