It's the field asking for the exact web address your visitors currently use to reach your live site (for example, www.yourschool.moe.edu.sg). It is not a new domain, it's the existing domain that will be re-pointed to your new Isomer Next site on launch day.
What should you do if you're unsure which domain to list as your "Expected Cutover Domain"?
Check the URL your live Classic site currently uses.