You need a partner account to mark an application as publisher verified. There isn't currently a test option without being a real partner. The prerequisite requirements list for publisher verification says, "The developer must have an Partner One ID for a valid Microsoft Cloud Partner Program account that has completed the verification process. The CPP account must be the partner global account (PGA) for the developer's organization."
Publisher Verification Requirements
How to become a Microsoft Partner
You can leave this feedback in the feedback portal though, and if you provide more details about your use case I would be happy to bubble this up as well. https://feedback.azure.com/
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