How to assign a person to a user field through the api.

If one has a field of this type in clarizen:

Field Type: Reference To Object
Referenced Entity: User

How can one assign a value to that through the api?

for all the other fields I have just passed a string of some value.

Having tried that with the above field I am getting the below message:

{'errorCode': 'EntityNotFound',
'message': "Entity '168205121' of type 'User' was not found",
'referenceId': '1uve0dLXgNOILNykrZKEXU'}

I have tried to pass in the:

username of the user

userid of the user

and the email of the user.

All result in the above error.