Is there way to create Guest User over API? We cannot find appropriate API.
The dashboard API does not allow you to add users as guests. The captive portal API allows you to authenticate guest WiFi users with a simple URL or a slightly more complex RADIUS authentication. You would then create your users on the RADIUS server.
Also, not via API, but Meraki allows guests to create their own guest accounts on the splash page. Perhaps that will work for your use case. More here: