Help with a Custom Function to copy field.

Help with a Custom Function to copy field.

I have two field in Leads Module. 
Field 1: Sales Agent
Field 2: First Sales Agent

I am trying to write a function that copies Field 1 in to Field 2. (I will use the function with Workflow, as soon as Field 1 is edited to execute the function)

Here is what I have so far, but the function is showing Error:

agent = zoho.crm.getRecordById("Leads",input.agent_id);

sales_agent = agent.get("Sales Agent");

if( sales_agent != "" ) 
{
leads_map = new.map();
leads_map.put("Sales Agent" , "First Sales Rep");
}

Please look at attached image of the screenshot. If someone can help with this function will be great. THANK YOU!!!