How to Add a User to a newly created Project in Workfront Fusion

I am trying to figure out how to add users to a project using Fusion. I have tried a Custom API call: Post and a Create Record for Project User. Neither work and produce a 422 error: [422] PRTU is not a top level object and can't be requested directly. I am trying to add a few users that will not be assigned to tasks to the new project and at the same time I need to remove the owner of the request from the converted request and add them to the project. Any help would be extremely appreciated!