generate token by passing private key and jsonstring

Try below:

using System;
using System.Security.Cryptography;
using System.Text;

public class TokenGenerator
{
    public static string GenerateToken(string privateKey, string jsonString)
    {
        try
        {
            byte[] privateKeyBytes = Encoding.ASCII.GetBytes(privateKey);
            byte[] jsonBytes = Encoding.ASCII.GetBytes(jsonString);

            RSACryptoServiceProvider rsa = DecodeRSAPrivateKey(privateKeyBytes);
            SHA1Managed sha1 = new SHA1Managed();

            byte[] hash = sha1.ComputeHash(jsonBytes);
            byte[] signature = rsa.SignHash(hash, CryptoConfig.MapNameToOID("SHA1"));

            return Convert.ToBase64String(signature);
        }
        catch (Exception e)
        {
            Console.WriteLine("Token generation failed: " + e.Message);
            return null;
        }
    }

    private static RSACryptoServiceProvider DecodeRSAPrivateKey(byte[] privateKeyBytes)
    {
        try
        {
            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
            rsa.ImportRSAPrivateKey(privateKeyBytes, out _);
            return rsa;
        }
        catch (CryptographicException)
        {
            throw new Exception("Invalid RSA private key.");
        }
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        string privateKey = @"-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----";

        string jsonString = "{\"UserID\":\"1\",\"UserName\":\"Ayyappa\",\"txnId\":\"123456\",\"customerMob\":\"8569896857\",\"lname\":\"lakshman\",\"name\":\"Balasubbiramani\",\"mname\":\"ram\",\"productCode\":\"DMT\",\"address\":\"Mumbai\",\"dateOfBirth\":\"05/10/1982\",\"agentId\":\"CUSTOMER123456\",\"partnerId\":\"4121\",\"kycStatus\":\"0\",\"pancardStatus\":\"0\",\"identityDocType\":\"0\",\"identityDocNo\":\"BgQkkDr14...\",\"channel\":\"4\",\"isOTPValidated\":\"0\",\"hashKey\":\"b2b7e6c8f6f011e2b6396c4d36d4ecd6\"}";

        string token = TokenGenerator.GenerateToken(privateKey, jsonString);
        Console.WriteLine("Token: " + token);
    }
}

Thanks