How to hide the phone field until another field is changed?

How to hide the phone field until another field is changed?

I want the phone field to be hidden until lead is claimed. Here is what I have so far:

if ( input.Sales_Agent == null) 
{
hide Phone;
}
else 
    {
show Phone;
    }

I am not good at programming but Zoho under Work Flow Automation is telling me that "On Load" is needed for "hide" function...

Please help how do I set this function in Zoho. I am doing all this under WorkFlow Automation.