SFTP using X.509 and .NET

I have a client that is using Tectia SSH and I am sending data over SFTP. However they require authentication via a X.509 privatekey/certificate. I have one I can use, but I'm not sure how to go about using the SmartFTP library to facility this.
Is it as easy as manually exporting the private key from the Windows Certificate store and loading from a file it in code? I have connected to other SFTP servers that use SSH Private/Public Keys fine using this method.
I haven't seen any samples that specifically address the usage of a X.509 privatekey/certificate.
Any help would be appreciated.

You can directly use the private key/certificate that is stored in the certificate store. Please contact support on how to set this up.

Thanks for replying.
How exactly do you contact support with just a Smart FTP Library license? Everything tells me to download the client and go to support option. When I do that it takes me to a page saying I need to have a license.
Michael Monte

You can contact support at smartftp.com and then provide your license/serial.