title: Fill Create form with default values from API call in react-admin
body: <p>I'm still quite unexperienced with React and react-admin. I searched a lot of resources but couldn't find any valuable hints. My use case is the following:</p>
  
  <h2>Status Quo</h2>
  
  <p>Here is an example of one of my <code>List</code> components. This provides the default button to create a new item which redirects the user to the <code>Create</code> view.</p>
  
  <pre><code>export const CountryList = props =&gt; (
    &lt;List
      sort={{ field: "name", order: "ASC" }}
      filters={&lt;CountryListFilter /&gt;}
      {...props}
    &gt;
      &lt;Datagrid&gt;
        &lt;TextField source="name" /&gt;
        &lt;TextField source="iso3" /&gt;
        &lt;EditButton /&gt;
      &lt;/Datagrid&gt;
    &lt;/List&gt;
  );
  </code></pre>
  
  <h2>Desired Behavior</h2>
  
  <p>Instead of an empty <code>Create</code> form I'd like to provide dynamic default values. If the user clicks on the "Create" button in the <code>List</code> view I'd like the following to happen:</p>
  
  <ol>
  <li>The application issues a custom GET request to an external server (outside the scope of the already defined dataProvider)</li>
  <li>The response data from that request is used to populate the <code>Create</code> form.</li>
  </ol>
  
  <p>While I know how to use e.g. axios to issue the GET request, I counldn't figure out:</p>
  
  <ul>
  <li>Where/on which trigger would I issue the request?</li>
  <li>How would I pass the reponse data to the form's default values?</li>
  </ul>
  
  <p>My idea was that I can somehow "overload" the default functionality of the create button.</p>