how can I register a route 53 domain for govcloud?

Amazon Route 53 domain registration is only available in the us-east-1 (N. Virginia) Region, not in AWS GovCloud. When you register a domain using Route 53, you must use the AWS CLI with the --region us-east-1 parameter or create your domain registration through the Route 53 console in the standard AWS partition.

To register a domain that you can use with GovCloud, you would:

1. Register the domain in the standard AWS partition using the Route 53 service in us-east-1
2. Use the register-domain command with the AWS CLI, specifying the us-east-1 region
3. Provide the required contact information in a JSON file

Once registered, Route 53 will automatically create a hosted zone with the same name as your domain and assign four name servers to it. If you need to use a different hosted zone than the one automatically created, you'll need to update the name servers for your domain to use those of the new hosted zone.

After registering your domain in the standard partition, you can then configure DNS settings as needed to work with your GovCloud resources.
Sources
Route 53 domain registration examples using AWS CLI - AWS SDK Code Examples
RegisterDomain - Amazon Route 53
Registering a new domain - Amazon Route 53