You just need to configure the domain on the SSID.
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After entering their own credentials, the user will then be prompted to enter their sponsor’s email. Users need to enter a sponsor email that matches one of the previously configured email domains. Using an undefined domain will return an error asking users to confirm the email.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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