Not sure about the possibility of creating an anonymous project?
As an unlogged-in user, you can create an anonymous project from a Knowledge Model preview link that the Owner of the Prject sent you. This project can be filled out and shared as a regular project. Although, in this mode you cannot create a preview or documents. In order to be able to you would have to log in.
Now you can work with the anonymous project as usual (fill out, share).
First, log in to your DSW account. Then go to the link of the anonymous project and click on the "Add to my projects" button.
The project is under your ownership now.