Npgsql Provide Client Certificate and Key - postgresql

Question

I am trying to formulate a connection to a PGSQL server that requires both a client certificate and key to operate.
First, I can verify connections to the Postgres database using SQLGate work. Provide host, User, password, port, database and mark Use SSL, then under SSL provide the Certificate and Key. The connection does not operate without either of those items. Using NPGSQL, I provide all but the key as for some reason NpgsqlConnectionStringBuilder does not contain a definition for some sort of client key.
var connectionString = new NpgsqlConnectionStringBuilder();
connectionString.Host = rInfo.Host;
int portNumber = 5432;
int.TryParse(rInfo.Port, out portNumber);
connectionString.Port = portNumber;
connectionString.Database = rInfo.dbName;
connectionString.Username = rInfo.Username;
connectionString.Password = rInfo.Password;
connectionString.SslMode = SslMode.Prefer;
connectionString.TrustServerCertificate = true;
connectionString.ClientCertificate = rInfo.CertFilePath;
//Poke the database, see if we can get in.
try
{
NpgsqlConnection npgsqlConnection = new NpgsqlConnection(connectionString.ToString());
npgsqlConnection.ProvideClientCertificatesCallback += provideCertificates;
npgsqlConnection.UserCertificateValidationCallback += validateCertificates;
npgsqlConnection.Open();
npgsqlConnection.Close();
return connectionString.ToString();
}
The exception is:
Error 28000 : connection requires a valid client certificate
Which is to be expected since I'm not providing the key anywhere. I have tried forcing the key to be added to the connection string via guessing:
connectionString.Add(new KeyValuePair<string, object?>("Client Key", rInfo.KeyFilePath));
But that's unrecognized. Libpq's PG Connect documentation labels it as sslkey, but that comes back as unrecognized as well. My best guess is using ProvideClientCertificatesCallback callback to provide the certificate, but I don't know how to have it pair with a key since it's just asking for an X509CertificateCollection.
The previous tool we were using was provided by Devart, but we have lost the license. We also will be connecting to a range of databases (with the same schema) instead of just one.
What are my options?