Change the radiobutton to Custom Domain and enter a valid domain (something like example.com or acme.io). No need to enter 'https' or 'www' in this field.
Click on Check Availability and click on 'Save Domain Name'.
changing domain to custom domain
Custom domains will be approved in less than 12 hours. To expedite, you can send email to hello[[[@]]]siteoly.com (remove square brackets)
DNS changes
Now, make below changes in your DNS settings (on your DNS provider like Namecheap, GoDaddy, Google Domains etc)
If you are setting your custom domain at root level (like yoursitename.com), please set up below two records in your DNS. Replace yoursitename.com with your actual domain name
If you need help, please login at https://app.siteoly.com and ping on the support chat window at bottom right corner or send email to hello[[[@]]]siteoly.com (remove square brackets)