to add role to organization, please use POST /management/roles/addRolesToOrganizationObject
For testing purposes it is probably the easiest to assign roles to groups and they will be then inherited by users in those groups
I just saw that swagger description is missing there (example&schema) and have created ticket to be corrected (ERA-9168).
Until it is done, you can go to management-studio and change role of a user, that way you will have a working example for the payload.
As for the uri/useruri issue, I have also created ticket for this to be corrected (ERA-9169)