Follow these steps to add subscript text in Notion:

1. Use the /math command to insert a math block. Block equation will display your text as a separate block.
2. Inline equation will display your text inline with regular text.
3. Write your text using TeX Notation.
4. Use underscore (_) for subscript, caret (^) for superscript, tilde (~) for space and curly brackets ({}) to group more than one character together.
5. Enter your properly formatted TeX text and click Done.
6. You now have your subscript / superscript text displayed on your Notion page.