Thank you for posting this in Microsoft Q&A.
As I understand you want to create a conditional access policy that will restrict MFA registration page only from device which is assigned to them.
This requirement is not possible as of now.
With conditional access you can create a policy to restrict or allow users to access particular apps based on devices platforms, IP addresses, device status, sign-in risks etc. But there is no filter as such to define policy using device owner.
However, if you are looking for this requirement then you can submit your feedback in Azure feedback portal in below link. This channel is monitored by our PM team directly.
https://feedback.azure.com/d365community/forum/22920db1-ad25-ec11-b6e6-000d3a4f0789
Let us know if you have any further questions.
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