How to get the refresh_token with getAcessTokenSilently method from the Auth0 React SDK

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I need to access to the refresh_token so I tried calling the getAccessTokenSilently method like this:

      authorizationParams: {
        scope: 'openid offline_access',
        grant_type: 'refresh_token',
      cacheMode: 'off',
      detailedResponse: true,

I can see in the networks tab that refresh_token is returned in the payload, but for some reason, getAccessTokenSilently returns everything except refresh_token. Why?