Sign up for secondary newsletter using Portal

I have two newsletters on my website. I want each newsletter’s respective feed to offer a dedicated signup button. After reading the docs, I know I can do this with a custom signup form, but can I do this while still relying on the Portal signup interface? I see that I can create Portal links for specific tiers, e.g. #/portal/signup/<tier>/monthly. Is it possible to create a similar link for a specific newsletter?