Guide

1. Inside your Figma design canvas, find and select the text layer you want to underline.

2. Following that, head over to the right-side panel and click the 'Type Settings' icon under Typography.

3. Find 'Decoration' from the list of options under the Basics tab.

4. Subsequently, click the 'Underline' icon.

5. After completing these steps, the underline effect will be added to the selected text layer.