gistfile1.txt

// getting the key
        private KeyClient KeyClient => new KeyClient(
            vaultUri: new Uri(_configuration["KeyVaultUri"]),
            credential: _getCredentialService.GetKeyVaultCredentials());

        public async Task<KeyVaultKey> GetEncryptionKey()
        {
            var keyResponse = await KeyClient.GetKeyAsync("encryption-key");
            return keyResponse.Value;
        }
        
// usage
        public async Task<string> Encrypt(string rawValue)
        {
            var encryptionKey = await _keyVaultService.GetEncryptionKey();
            var cryptoClient = new CryptographyClient(encryptionKey.Id, _getCredentialService.GetKeyVaultCredentials());
            var byteData = Encoding.Unicode.GetBytes(rawValue);
            var encryptResult = await cryptoClient.EncryptAsync(EncryptionAlgorithm.RsaOaep, byteData);
            
            return Convert.ToBase64String(encryptResult.Ciphertext);
        }