You can easily make text bold or italic in XD, but not underlined. There should be an Underline style or toggle button for creating designs with links, etc.
You can now underline text in the December release of Adobe XD. If you’re interested in improvements to the underline feature (like adjusting the distance between the underline and the baseline), please do feel free to file that as a separate ticket!
Joachim Tillessen commented
Take note: Underline Text alone does not enable to implement Hyperlinks. Since it is currently not possible to select a text string in an textblock in Prototype mode.
Nathan Snelgrove commented
I think this is really a key part of building intuitive pages - we need text links, that's how the web started and a key part of contextual movement through web pages.