Change default author to Firstname Lastname

I run a website where we create several users every day who publish content. Right now, the WordPress default is to use their username as the display name.

I need to change the default display name to Firstname Lastname, either automatically or through a dropdown in the Create User screen.

I’ve searched on this board and elsewhere for solutions but there doesn’t seem to be one. Can any experts out there help me out?

Thanks!