So here you have, a quick-and-dirty Get-CertificateFromCredential function you can use to get the certificate for the credentials the user selected from the drop down in the Get-Credential window:
Then, you can use the function:
$cred = Get-Credential -Message 'Select the SMARTCARD' Get-CertificateFromCredential -Credential $cred
Important: Keep in mind that Get-Credential cmdlet doesn't verify the credentials anywhere, it just opens the $Host.UI.PromptForCredential popup and returns a PSCredential object. The credentials themselves are verified only when used with another cmdlet.
This means that the user can select a certificate from the dropdown in the Get-Credential window, enter an incorrect PIN and this function will still return the certificate.