Normally users can login using desktop apps using device authorization code without having to get admin approval. The administrator optionally can configure FileCloud to require user's new devices to be approved by admins before being allowed to be used to login into FileCloud.
To require admin approval, select the policy option in Settings→Policies→Global Defualt Policy. Note that admin approval can be disabled by policy for some user accounts by creating a separate policy.

When this option is enabled, when a user account attempts to login with a new device, the request is sent for approval by the admin. The device shows up in the Devices tab in the Admin portal.
Select Permissions and select the "Allowed" option to approve the device.

Once approved, the user can now login into their account and approve the device to get the device authorization code as usual.
Note that once the device is approved, the user can re-attempt the whole device code based authentication flow without re-requiring admin approval.