Currently there are no APIs that allow you to directly claim or unclaim an Atlassian account.

This means that, for organisations that for operational reasons cannot automatically claim all accounts on their domain, you need to build a process that involves manually accessing https://admin.atlassian.com and uploading a csv file containing new accounts you wish to claim.

Ideally there should be a couple of endpoints in the Organizations REST API that would allow them to instead claim or unclaim accounts instead.

There is one known workaround for the lack of a claim call and that is:
1. Provision the user via the User Provisioning API (As this automatically claims the user if possible)
2. Immediately delete the user from the SCIM DB afterwards via the Delete Only In SCIM DB endpoint in the same API