Does anyone know how I can add my custom designs to the "New Design" interface? See screenshot below
You can't, what you can do is create your designs in BSS and then save your projects and reuse them as needed. I believe they talked about adding more of the startup designs quite some time ago, but I don't know if that's still in the plans or not.
Thanks for the feedback Jo, we are considering to use this took with a team and having a lead create Templates would save the step of sharing designs manually.