On Load set variable of User lookup field

On Load set variable of User lookup field

Hi - 
I have a task form that includes a "Users" look-up field set to display the all user's Email Id.  I'd like to set this variable to the logged in user but still allow the option to select another user manually if need be.
Under workflow I have added the following code under On Load:

  1. input.User = zoho.loginuserid;
However, it is not selecting the logged in user or anything else....any ideas?  Is it perhaps because email addresses are being used instead of username?
Thanks.