Original: For example, to create a link to a subpage you want to be called "Sandbox," place the text [[/Sandbox]] on your user page.
Simplified: You can "quote" categories by adding a colon before "Category", like this: [[:Category:Bridges]]. If you wanted to create a user subpage called "Sandbox", you would find it here. To link to a user subpage called "Sandbox" from your main user page, place the text [[/Sandbox]] On your page, or use a piped link with the same source.