This can be done from your User Area > My Accounts > Manage (next to the domain name you want) > Your Domain DNS.
Enter the custom name servers generated for you by Ezoic (these can be found under 'Settings' > 'Site Integration' > 'Name Servers' > 'View Instructions' in your Ezoic Dashboard) and then click the Update button to save the changes. It will take several hours for the DNS change to propagate.
Note*: When Ezoic requests a copy of your site from your Siteground server for us optimize, their Anti-Bot AI may block our requests. You can read more on their Anti-Bot AI here. As such, we recommend getting in touch with SiteGround support and asking them to disable this feature for your site.