Creating defect with project or user as a vaiable

Creating defect with project or user as a vaiable

I'm trying to programatically create a defect. When I pass the project or
user fields as a variable to the JsonObject I get the following error:
Cannot parse object reference from ""/user/2.........""
Code:
newDefect.addProperty("SubmittedBy", username);
Where username = "/user/2........."
but if I have the following code:
newDefect.addProperty("SubmittedBy", "/user/2.........");
it passes. I would like to be able to have the program dynamically look up
the user and be able to get the reference, but so far when I have tried I
get that error.
Any ideas?