Problem mapping a subdomain to an Aggregate site in GAE

Hi, everyone.
I have an GAE Aggregate project in at the url projectname.appspot.com
Now, I bought a domain mydomain.com from LWS and I want to map the subdomain projectname.mydomain.com to projectname.appspot.com
After setting up succefully all the required steps suggested by GAE (https://cloud.google.com/appengine/docs/standard/python3/mapping-custom-domains), I'm able to reach my GAE app via projectname.mydomain.com; but, once the page finishes loading, instead of the
new url (my hope), the old one (projectname.appspot.com) still displaying in the address bar of my browser.
PS:
I'm aware that the present post is not a specific ODK post: I apologize for that
but I'm asking you in case somebody would had faced and solved such an issue.
In advance, thanks for your help.
Amal