DPML override works on a domain that is subject to claims.
Trying to do this in the portal we had to complete this steps:
1. Fill in details: domain name, contacts, etc.
2. Click Register
3. Notification displayed to user that domain name is subject to claims
4. Fill in TCN information (noticeID et.)
5. Click Register
6. Notification displayed to user that domain name is blocked and SMD file is required for override
7. Attach SMD + click Register.
For EPP this operation is done in one command (attached).