title: Formstack API - Field with multiple checkbox values as array
body: <p>I create new submission for the specified form.
  I read documentation on <a href="https://developers.formstack.com/reference/form-id-submission-post" rel="nofollow noreferrer">https://developers.formstack.com/reference/form-id-submission-post</a>
  I tested a lot of methods but nothing work. Only 'string' works fine (one value). I don't know where is error in my array.</p>
  <pre><code>// one value as string
  'body' =&gt; json_encode([
      // x - is a field id
      'field_x' =&gt; 'option-1',
  ]),
  
  // solution 1
  'body' =&gt; json_encode([
      // x - is a field id
      'field_x' =&gt; [
          '1' =&gt; 'option-1',
          '2' =&gt; 'option-2'
      ],
  ]),
  
  // solution 2
  'body' =&gt; json_encode([
      // x - is a field id
      'field_x' =&gt; [
          'option-1',
          'option-2'
      ],
  ]),
  
  // solution 3
  'body' =&gt; json_encode([
      // x - is a field id
      'field_x' =&gt; [
          1 =&gt; 'option-1',
          2 =&gt; 'option-2'
      ],
  ]),
  </code></pre>