Obsidian.md doesn’t come with an underline feature built in. However you can install the Obsidian Underline plugin from the community plugins dialogue of your Obsidian.md application. This plugin adds the Ctrl + U shortcut and inserts the HTML markup (not markdown) for an underline. The result is an underline. Unfortunately it looks messy in raw markdown, but it’s currently the fastest and best way that I know of to add an underline to an Obsidian.md markdown document.
<ul> This is some underlined text </u>