How could I encrypt some parameters with Rijndael algorithm?

Question

I want to encrypt the following parameters:

• user
• pass

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope>
   <soap:Body>
      <user>User</user>
      <pass>Pass</pass>
   </soap:Body>
</soap:Envelope>

The ideal final XML document would be something like this:
(The values are random letters with no meaning just to show you what I want to do)

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope>
   <soap:Body>
      <user>gsdfhnsafjjetjte</user>
      <pass>herahrejhtjhsteajt</pass>
   </soap:Body>
</soap:Envelope>

• I have to use Rijndael in order to encrypt , but I can not figure out how to do this.
  
• I have already checked the Microsoft's Documentation about Rijndael , but I can not find a way to implement this in my code.
  
• I have also checked a lot of articles and resources , but none of this helped me.
• I have to send it back to the XML document as a string , as you can see in the above code , but the documentation for Rijndael returns byte[].

IMPORTANT
I must use as a Rijndael Key a token from an Ouath 2.0 API. How can I use this and where in the Rijndael code?

EDIT For example: I have to encrypt with Rijndael the below parameters in the method:

APISoapClient soapClient = new APISoapClient ();

string id = "callID";
string username = "user";
string password = "pass";
string data = "someData";

string test = soapClient.getInfo(id, username, password , data);

Could anyone provide me with a sample of code which encrypts the above using the Rijndael algorithm?