Adding a custom domain for your application in Google cloud Platform

Purchase a new domain, unless you already have one that you want to use.

follow the instruction mentioned in the link below.

https://cloud.google.com/appengine/docs/standard/python/mapping-custom-domains

Ensure that your domain has been verified, otherwise, you will not be able to proceed with the above steps.

you can verify your domain on the link below.

https://www.google.com/webmasters/verification/home