You should understand what needs to happen from the domain registration perspective when launching a new microsite on a new sub-domain URL.
Creating a microsite and deploying it on a new subdomain URL does not require a new domain registration; however, a public DNS entry is usually required. The new subdomain will require a public DNS CNAME record of the same value as the pre-existing live domain (or an A Record potentially, but usually a CNAME). This is done either in the interface for the registrar, or some DNS service such as Route53, whatever is being used for public DNS. Note that a DNS record update must be completed by the Optimizely team. You must submit a support ticket to fulfill this request.
The server on which the site lives will also need a new binding set up to accommodate the new sub-domain. This is set up in the IIS management console.
SSL also must be considered. Often times a wildcard cert is used, or a UCC SAN SSL certificate for multiple domains. That cert would then live on the load balancer, or IIS server if there is no load balancer.
The database will also need an entry for the new sub-domain.
More information on how to create a microsite using site copy can be found in this article: Creating a Microsite Using Site Copy
For details on microsites in Spire see Microsite support in Spire.