You can do it like this.
Step 1: Website > Add site
Step 2: Add the domain panel.domain.com.
Step 3: Go to the settings of the domain you added.
Step 4: Click on "Redirect" in the left menu.
Step 5: Click on the "Add redirection" button.
Step 6: Click on the domain you added from the "Source" section.
Step 7: Type the current panel address as "domain:8888" in the Target URL section.
When you go to panel.domain.com, it automatically redirects you to domain:8888 port.