After you create the sub-site, go into the Site Settings (via the Network Admin) and change the URL for the sub-site to https://event2.domain.com .
That’s the same way that you would assign another domain name to a sub-site. For example, if the main site is www.domain.com, sub-site 1 would be www.domain.com/subsiteone (you decided the subfolder name). That is what you use when you create it.
Then change www.example.com/subsiteone to event2.domain.com . Since your hosting nameservers know about ‘domain.com’, that shouldn’t require any additional configuration changes at the hosting level.