There are a few potential options I think:
1
For the interaction without registration - you can probably try to adopt Custom Wizard plugin (I’m not sure if they have this feature already), but I think it can be possible to request it on marketplace.
2
As @angus mentioned in the same post:
We’re actually about to release a “landing pages” plugin for discourse which includes a contact forum module, which is better suited to this use case. Stay tuned for that.
The plugin exists but I’m not sure if this feature is ready - probably you can already use contact form from published Landing Page.
3
It’s possible to create a special user on your forum and create posts for not registered users by API - f.e. from Google Forms or any other service.
I can help with that.