//  Here is how I use it in my AppModule.java, note there may be no any HTTP Request when this code executing:

import xxx.web.pages.oauth.Callback;

// ...

    @Contribute(OAuthServiceCatalog.class)
    public void contributeOAuthServices(MappedConfiguration<String, OAuthServiceConfig> configuration,
            // ...
            OfflineLinkSource offlineLinkSource)
    {
        //  ...
        Link callback = offlineLinkSource.createPageRenderLink(Callback.class);
        callback.addParameter(Button.SERVICE, XXX_EXTERNAL_SYSTEM);
        
        String uri = callback.toAbsoluteURI();
        // ...