How to programatically update User field in a form

How to programatically update User field in a form

See the sample code below:
    for each o in New_Order  [Order_Number == "352"]
    {
        info o.Order_Number + " " + o.Owner_Name + " " + o;
        o.Owner_Name = zoho.loginuser;
    }

Owner_Name is a User field in the New_Order form.

I tried multiple ways to update it but all failed:
1) zoho.loginuser
2) zoho.loginuserid
3) hardcoded string of my user name "lexshan8"
3) hardcoded string of my email " lex.shan@walkintour.com"

What is it expecting? The error message is not helpful at all.