Custom Card Integration - Where to begin

So if I understand correctly, I must use a public application. We are not using a cloud provider so I don’t think serverless functions are an option, is that correct? Now, I created a public app locally, but I cannot upload it to HubSpot. It’s telling me I don’t have access to the beta. I have followed the quickstart (no mention of which beta and how to add it). I’ve been in the Product Updates section and am blindly scrolling through available betas. Any idea which one I need? I see a few related to UI Extensions but it already says everyone is a member of the beta. Here’s my error,
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