official_sitecore_install_script.ps1

#install client certificate for xconnect
$certParams = @{
    # EDIT: Remove "_client" from the CertificateName"
    CertificateName = "$prefix.xconnect"
}

...

#deploy xconnect instance 
$xconnectParams = 
@{ 
    # EDIT: Add SSLCert to the existing parameters
    SSLCert = $certParams.CertificateName
}

...