Pressbooks installations and books hosted at pressbooks.com now come with the Hypothesis plugin automatically installed. If you are working with Pressbooks, all you have to do to activate Hypothesis is to go to Settings and configure the app however you want. Designated pages will be annotatable natively: the sidebar will appear on the page, and text selection will trigger the app.
You can configure Hypothesis highlights to appear automatically. You can configure the sidebar to open automatically. Both these actions can of course be done manually if you choose not to make these settings the default.
You can also configure Hypothesis to be activated on PDFs hosted within the Pressbooks book. (Note: PDFs must be OCRed to be annotatable using Hypothes.is.)