If it's not already in front, click the cPanel tab to make it active.
Scroll down and find the Domains section. Then click the DNS Zone Editor icon.
Look for the Add DNS Record section and do the following:
For Host Record, type the name of your desired subdomain (for example, mylandingpages if you are trying to create mylandingpages.mysite.com).
Leave TTL unchanged.
For Type, choose CNAME.
For Points To, type pages.optify.net.
When you're done, click Add Record.
Your changes will appear below in the CNAME (Alias) section. You can click Edit on the appropriate row if you need to change it later.
Setting your custom domain name in Optify
Now that you've made the necessary DNS changes with your domain registrar, you need to customize your landing page links in Optify. To do that, see the instructions in Selecting your custom domain in Optify.