How to get private key from rsa and dont know how to attach it the code of itexshpar.dll function

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.NET3.5

.NET

I am using itextshpar.dll to attach digital signature to pdf.
Using epass2003 token which is having non-exportable private key.

Getting .cer file and converting it into .pfx file without privatekey.
Than login to token using below code.

System.Security.Cryptography.CspParameters csp = new System.Security.Cryptography.CspParameters(1, "EnterSafe ePass2003 CSP v1.0");
csp.Flags = System.Security.Cryptography.CspProviderFlags.UseDefaultKeyContainer;

System.Security.Cryptography.RSACryptoServiceProvider rsa = new System.Security.Cryptography.RSACryptoServiceProvider(csp);

// Sign the data using the Smart Card CryptoGraphic Provider. 
byte[] sig = rsa.SignData(certData, "SHA256");
How to get private key from rsa and dont know how to attach it the code of itexshpar.dll function