Go to a page that has not been created. (example: https://powdertoy.co.uk/Wiki.html?id=random) then click create page.

What ever you'd like to contribute would go under one of the classes that has already been fabricated, it would justify such that anything of matter would go appropriately under its correlated section, since TPT is fully understood within its realm of operation, everything has been created for it with little obfuscation.