@Ruslan Gilmetdinov Thank you for reaching out to Mucrosoft Q&A.
I understand that you are having issues with verifying the onmicrosoft.com domain. You can't verify a .onmicrosoft.com domain since that's owned by Microsoft. You can only verify your own custom domain (i.e. testdomain.com).
I think the issue that you are facing is similar to this article here- https://www.michev.info/Blog/Post/3173/adding-multiple-onmicrosoft-com-domains-in-the-same-tenant
You can add the onmicrosoft.com domain but the verification part is not available yet.
Hope this helps. Please let usknow if you have any further questions/concerns and we will be glad to assist further. Thank you!
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