How do I underline text in After Effects?

Know this was posted ages ago, however, a workaround I use is to make a line mask on the text layer where I want the underline to be and then apply a stroke effect.

So:

1. Use the pen tool to make line below text as a mask

2. Effect > Generate > Stroke or type stroke in the effects and presets search bar and apply it to the same text layer

3. Set path as Mask and adjust other settings as desired in the Effect Controls panel.

The simple answer is that you can't.  The After Effects text engine does not have an underline feature.

But, as with most things in After Effects, there are a million ways to do things. 

You can simply add a solid layer, draw a path under the text, and apply the stroke effect.

You can draw a shape layer as a line under your text.

You can overlay a second text layer and use underscores to underline your text.   ________________________

While perhaps inconvenient, all of these solutions offer you the advantage of being able to independently animate the underline, because it is not built into the text itself.

Discussions of this topic have happened for years on this forum, and generally devolve into a partisan "Why can't I underline text?" vs "Why would you want to, it's bad typography!" argument.