usually your client will know his future domain name beforehand. so you could do it the other way around.
you create the future web-domain of the client directly in the panel when starting and put the working name / sub domain just as alias into the config. if you want to setup an ssl cert for that subdomain while developing, you can always run letsencrypt for just that domain manually and put it in or get a free three month gogetssl or whatever…
when you go live you just need to point the dns record of the clients domain properly to your server get a new correct ssl cert and adjust the base url in wordpress, done.