The Registration ID in this case is your DevPost user name. This is how you can locate your registration ID or DevPost user name.
1. Sign into https://devpost.com/ 2. Click on the person icon / your picture icon on the right side of the top header and select Portfolio from the drop down list.
3. You will see your portfolio or profile – the information mentioned next to your name in parenthesis () is your registration ID for this Hackathon. For e.g. for me it's preetikr (it also shows up for participants when you click their name and go to details).