Maybe I'm unique but I've been in Product Marketing for a long time and I've never been asked to present a "marketing portfolio". I have, on the other hand, been asked to present on specific topics or share specific experiences on many occasions.
If someone is asking you to present a "marketing portfolio" and you are new to product marketing, I would ask the person asking you to provide more detail on what they are looking for.
If you don't have a portfolio, then I would tell them that you are very interested in the role and I would ask them whether or not they can think of a specific exercise you could do to prove you have the right skills for the role.
For hiring I've done, I always have candidates go through an exercise designed to prove that they are a good fit for the role. The exercise usually involves either relating past experiences relevant to the role, or to analyze content and make recommendations for improvements. In some of the roles I am hiring for now, I am asking candidates to tell me what they like and dislike about our messaging versus a few key competitors, and what changes they would make and why.
If you were previously on the creative side, or the campaign side of marketing, then maybe they are looking for previous examples of your work? Hard to say without more info.