Getting current user Id

Getting current user Id

Hello, everybody! 
I want to set the login (current) user as a potential owner. I'm trying to do this with the custom function. Please help me with this task. I now that I have to update field  "SMOWNERID" to current user Id, but I can't understand how to get ID. I tried to use system variables zoho.loginuser and  zoho.loginuser but none of them doesn't return the ID.