Hello Eric,
First of all sorry for my late reply.
You understand the issue; mij public internet website is http://www.xxx.nl and if I look in Office 365 administrator the adres of my sharepoint website is http://xxx.sharepoint.com if I click change adres then I get an error that the process of changing the default domain name (website) to my companies domain name (website) is still ongoing.
I don't understand what you mean with: "If so, I want to confirm that if users click Member Login in your public website, the URL begins with your custom website address or default website address?"
I don't use the default website address because I changed it to my own domain name but in this way Iam not able to change my own website within sharepoint.
Best regards,
Rene Weber