title: Setting custom key when pushing new data to firebase database
body: <p>Well, I am new to Firebase and I want to have my own keys while pushing new data to database. </p>
  
  <p>Problem:</p>
  
  <pre><code>FireBase.push().setValue(mapped_values);
  </code></pre>
  
  <p>This gives structure like below:</p>
  
  <p><img src="https://i.stack.imgur.com/i9VCG.png" alt=" database output"></p>
  
  <p>How can I create my own custom key there?
  Such as username or something.</p>