Specifies the refresh token received from or sent to the authorization server.
When GetAuthorization is called if the authorization server returns a refresh token along with the access token, this property will hold the refresh token. Save this value for later use.
When your access token expires, set this property to the corresponding refresh token. Then call GetAuthorization and the class will use this token to retrieve a new access token. The new authorization string will be returned by the GetAuthorization method. No user interaction is required when refreshing an access token.