In global settings administrator should be capable of adding custom domain.

All shared pages should be available under defined custom domain.

Our user should point DNS CNAME to jira.external-share.com.

We should discourage usage of A record. Technically jira.external-share.com points to 51.77.42.240 but we do not guarantees that this might change in future.

if user would like to point root domain then they should use dns flattening.