After months of searching, I finally found a way to disable the startup popup window in CorelDraw 2017. My version is 184.108.40.2069, but it should work for other versions as well.
So what's the secret that even tech support won't tell you?
You have to run REGEDIT as Admin and then look at this branch:
HKEY_CURRENT_USER \ Software \ Corel \ CorelDRAW \ 19.0 \ IPM_CDGS \ PremiumFeatures \ sub
- Doubleclick on LastLaunchCount and change the value to 3
- Doubleclick on Schedule, delete its value and type: 10 00 00 00 01 00 02 00 ff ff ff ff ff ff 01 00
- That should be it, the popup is now gone!
Here's what that registry branch should look like after those changes: