Q: Does Webstarts have a default web-address for new websites so that you can prototype or create websites ...
without having to assign a domain to them?
Brizy cloud does this.
Vultr.com does this.
For example: mywebsite.webstarts.com

You are a verified purchaser. It would be very easy for you to find the answer just by creating a website.
If you create sites for clients in the Designer Platform, you are asked to name the site. The temporary address until you assign a domain will be https://my.secure.website/[name-of-site].
If you create a site in your own dashboard, you will be asked to either connect a domain OR choose a subdomain. The URL for the site will be https://[name-of-site].webstarts.com, and you can change it in the dashboard. If you don't write a site name. You can also click "Choose later" and a number will be generated as part of the URL, like https://754716176664084828.webstarts.com. You can change it in the dashboard.
The help center is also a great resource when getting started with WebStarts.
It have and it is dot webstarts.com Hope that can help you !