You may have noticed that displayName is not available from ‘Search and Logon Options’ tab under General Settings in the PMAdmin site.
This is by design. An enhancement request (TF00629535) has been created detailing the feature: Add "Display Name" attribute or possibility of choosing any other attribute to display on account search options.
The product team will evaluate the request and this feature may become available on a future release of the product.
Please refer to this article for updates or contact support referencing the Enhancement Request ID: TF00629535.