With the introduction of HTML Additions, the ability to load an external script is no longer limited to the public P5.js studio. The public Processing.js-based studio.sketchpad.cc and select private studios now provide a way for you to load external libraries, scripts, HTML, CSS, etc. This has long been a top request, so I'm happy to finally make it available.
How do I load libraries?
The mechanism is the same, regardless of the library. If you're logged in and working on a sketch in any Sketchpad studio, you'll see an HTML link in the editor. Clicking this link opens a dialog that lets you input arbitrary HTML. Feel free to include jQuery, Paolo Pedercini's p5.play, something from the p5/D3 Cookbook for P5.js, ToxicLibs.js (for Processing.js), or just add some DOM elements to manipulate and CSS to style them.