A custom field be filled automaticaly once another filed value is updated (or created)

A custom field be filled automaticaly once another filed value is updated (or created)

Hi:

Have a question:

For example, in Contacts Form, we have [First Name] and [Last Name] fields, now, for some reason, we created a new field called: [Customer Name]. (mapping the value with Quickbooks' Customer Name field).

Is there anyway that I can setup a "Rule" that I can update the [Customer Name] field once the First Name and Last Name are created (or updated)? 

Seems simple, but I don't find anywhere.

Thanks.