How can I add a button to Request Tracker GUI that triggers a custom action?

I'm working with Request Tracker and I would like to add a button when you visualize an open ticket that triggers a POST call to an external Java Server and it returns a PDF file with a report about the ticket. What is the best workaround?

There are many different ways to add this sort of customization. The easiest might be to write some Javascript to add the button by looking at the divs and ids RT writes to the ticket display page. You can drop the Javascript right in the styling box on the Theme Editor page.

If you can request the PDF via GET, you can just add a link in the Links section. Or, depending on what you're trying to do, you can also do dynamic things with custom field values as described in the External Custom Field docs.