title: LoopBack 4 - Add 'Authorize' button in API explorer
body: <p>I'm new in LoopBack v4 and I'm trying to send an authorization key with every request.
  I used Swagger before, and I used to add the api key, after clicking on the "Authorize" button to the right.
  To avoid error due my inexperience, I've started with a fresh app of "Todo List" example.
  I tried (with not success) to follow what this post suggests: <a href="https://stackoverflow.com/questions/54362172/nodejs-loopback-4-add-bearer-token-config-into-swagger-explorer">Nodejs Loopback 4 add bearer token config into swagger explorer</a></p>
  
  <p>What I did is modify the file <code>src/index.ts</code> with this code:</p>
  
  <pre><code>export async function main(options: ApplicationConfig = {}) {
    const spec: OpenApiSpec = {
      openapi: '3.0.0',
      info: {
        title: 'LoopBack Application v2',
        version: '1.0.2',
      },
      paths: {
      },
      securityDefinitions: [
        {
          api_key: [
            {
              type: 'apiKey',
              name: 'api_key',
              in: 'header'
            }
          ]
        },
      ],
    };
  
    const app = new TodoListApplication(options);
    app.api(spec);
  
    await app.boot();
    await app.start();
  
    const url = app.restServer.url;
    console.log(`Server is running at ${url}`);
    return app;
  }
  </code></pre>
  
  <p>Basically, I added this line: <code>app.api(spec);</code> and the config <code>securityDefinitions</code>. The <code>spec</code> constant changes the <code>title</code> and <code>version</code>, but I still cannot see the "Authorize" button. For sure, I'm missing something or doing something wrong.</p>
  
  <p>Any help is appreciated! Thanks!</p>