Combining 2 fields on a form

Summary

automatically fill in a field

Content

Hello,

I have a business object called customer.  It has 3 fields.  Client, First Name, Last Name.  When a user fills in first and last name on the ADD form I would like the "client" field to be automatically populated with "Last Name" "," "First Name". So: "Smith, Bob".  I know that the client field has to be read only but what about the rest?

Thank you in advance!