title: SlackBot OpenModal Error: "Missing Charset"
body: <p>I want to make an api Call via Zapier to open a Modal in Slack.</p>
  <p>But I always get the error:</p>
  <pre><code>ok: false
  error:  invalid_json
  warning:    missing_charset
  response_metadata:
  warnings:
  1:  missing_charset
  </code></pre>
  <p>This is my Request Body:</p>
  <pre><code>{
  &quot;token&quot;:&quot;XXXXXXXXX&quot;,
  &quot;trigger_id&quot;:&quot;XXXXXXXXXX&quot;,
  &quot;dialog&quot;: {
    &quot;callback_id&quot;: &quot;projekt-verantwortliche&quot;,
    &quot;title&quot;: &quot;Projektverantwortliche auswählen&quot;,
    &quot;submit_label&quot;: &quot;Request&quot;,
    &quot;state&quot;: &quot;Limo&quot;,
    &quot;elements&quot;: [
      {
        &quot;type&quot;: &quot;users_select&quot;,
        &quot;action_id&quot;: &quot;projekt-projektleiter&quot;,
        &quot;placeholder&quot;: {
           &quot;type&quot;:&quot;plain_text&quot;,
           &quot;text&quot;:&quot;Projektleiter auswählen&quot;
          },
      },
       {
        &quot;type&quot;: &quot;users_select&quot;,
        &quot;action_id&quot;:&quot;projekt-berater&quot;,
        &quot;placeholder&quot;: {
           &quot;type&quot;:&quot;plain_text&quot;,
           &quot;text&quot;:&quot;Berater auswählen&quot;
          }
      }
    ]
  }
  }
  </code></pre>
  <p>What am I doing wrong?</p>
  <p>Here a Screenshot of the whole call:
  <a href="https://i.stack.imgur.com/F4QwP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/F4QwP.png" alt="enter image description here" /></a></p>