For the community self registration page if the user enters the detail, I want to check if the contact already exist on the basis of email entered by user. If it does, I want to create user for that existing contact. How can achieve this?

I have a requirement where I need to get Base URL in managed package class. I am using Url.getSalesforceBaseUrl().toExternalForm() to get base URL but getting
https://custom-domain-name--prefix.ap2.visual.force.com instead of https://custom-domain-name.my.salesforce.com 
Thanks in advance.

For the community self registration page if the user enters the detail, I want to check if the contact already exist on the basis of email entered by user. If it does, I want to create user for that existing contact. How can achieve this?

I have a requirement where I need to get Base URL in managed package class. I am using Url.getSalesforceBaseUrl().toExternalForm() to get base URL but getting
https://custom-domain-name--prefix.ap2.visual.force.com instead of https://custom-domain-name.my.salesforce.com 
Thanks in advance.