While you are still in the early phase of your career, I would advice you genuinely contribute to Open source software and Stackoverflow. Work on building and growing your personal projects.
Everything I submitted in my application was done in my own personal capacity. I got endorsed.
Just like you, I am introverted. Good thing, you must not be a public speaker to qualify.
Search the forum to learn more about the type of documents[evidence] successful applicants submitted.