protected String addExchangeToken(final String url, final OAuth2AccessToken accessToken) {
        val profileDefinition = (FacebookProfileDefinition) configuration.getProfileDefinition();
        val facebookConfiguration = (FacebookConfiguration) configuration;
        var computedUrl = url;
        if (facebookConfiguration.isUseAppsecretProof()) {
            computedUrl = profileDefinition.computeAppSecretProof(computedUrl, accessToken, facebookConfiguration);
        }
        return CommonHelper.addParameter(computedUrl, EXCHANGE_TOKEN_PARAMETER, accessToken.getAccessToken());
    }
Adds the token to the URL in question. If we require appsecret_proof, then this method
will also add the appsecret_proof parameter to the URL, as Facebook expects.

@param url the URL to modify
@param accessToken the token we're passing back and forth
@return url with additional parameter(s)
pac4j-oauth/src/main/java/org/pac4j/oauth/profile/facebook/FacebookProfileCreator.java