Follow the below steps to setup a custom domain for your project. This works with any paid plan.
Login to https://app.siteoly.com/
Mouse hover on the site card and click Edit.
Click on the Edit icon on Site Domain Name
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'.
Now, make below changes in your DNS settings (on your DNS provider like Namecheap, GoDaddy, Google Domains etc)
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)