Encoding in RSA in C# for Symfony(PHP)

Question

I want to make an application, in Xamarin, with a connexion. So, I want to encrypt my password to not be vulnerable on the network, and then encode it in base 64. I made it in Symfony to decrypt my password :

/**
 * @Route("/Decrypt", name="Decryption")
 * @Method({"POST"})
 */
public function Decryption(Request $request){

   $passwordEncrypted = $request->headers->get("x-password");
   $rsa = new Crypt_RSA();

   $passwordEncrypted = base64_decode($passwordEncrypted);

   $pr_key = file_get_contents("private.key");
   $rsa->loadKey($pr_key);
   $password = $rsa->decrypt($passwordEncrypted);

   $data = $this->get('jms_serializer')->serialize($passwordEncrypted, 'json');

   $response = new Response($data);
   $response->headers->set('Content-Type', 'application/json');

   return $response;
}

But I don't know how to encrypt the password in my Xamarin side. I found few samples on Internet but no explanation with them. For the moment, I have :

    public partial class ConnexionPage : ContentPage
    {
       private void Connexion(object sender, EventArgs e)
       {
          byte[] publicKey = Encoding.ASCII.GetBytes("/*I insert my public key*/");
          byte[] testData = Encoding.UTF8.GetBytes("test");

          RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();

          RSAParameters RSAKeyInfo = RSA.ExportParameters(false);

          RSAKeyInfo.Modulus = publicKey;
          RSA.ImportParameters(RSAKeyInfo);

          byte[] encryptedData = RSA.Encrypt(testData, false);
       }
    }

I just do a test for the moment but I get a result : A byte Array with a size of 280. But when I convert it in base64 and test it into my Symfony side, i don't fall back on "test".

Can someone help me please?