Placeholder for customized registration fields

Hi,

I am working on 8.8 Micro Jobs and created two registration fields – First Name & Last Name.
I want to add a placeholder for both of this fields and found that they can be edited from the class_core.php file. The problem is that these two fields are populated in the class_core.php file together, that way adding a placeholder will affect both of them.
Any suggeations?

Thank in advance.

Regards,
Yaniv