How to verify publisher for Azure AD B2C app

Question

I have an AD B2C app on a separate directory / tenant from my corporate tenant, which I am using in a custom policy to enable external users to sign in.

However, when signing in the user is prompted that the app is unverified:

How do I verify the application's publisher?

I have posted a question to stack overflow here with more details on how my app is configured: https://stackoverflow.com/questions/76303452/how-to-verify-publisher-for-azure-ad-b2c-app

When trying to verify the publisher, I get the following error:

{
    "error": {
        "code": "MPNAccountNotFoundOrNoAccess",
        "message": "The MPN ID you provided (1234567) does not exist, or you do not have access to it. Please provide a valid MPN ID and try again.",
        "innerError": {
            "date": "2023-05-22T05:40:35",
            "request-id": "539cdf4d-a6cf-415b-a7ea-6fe020332043",
            "client-request-id": "de0016fc-bc14-4780-8391-da6d4d937bf9"
        }
    }
}

The suggested remediation for this error at https://learn.microsoft.com/en-us/azure/active-directory/develop/troubleshoot-publisher-verification#mpnaccountnotfoundornoaccess says to ensure that the tenant the app is registered in is associated with the MPN account, but this does not seem to be possible.

What steps am I missing here, to be able to verify the app publisher?

Thanks!