title: DocuSign API: custom fields populating
body: <p>I'm confused about the difference between "tabs" and "custom fields," perhaps --</p>
  
  <p>When I send a json to DocuSign API's baseURL/envelopes, asking the API to send an envelope with a template, it works fine:</p>
  
  <pre><code>{ "accountId": "xxx",
     "status": "sent",
     "emailSubject": "Please sign this document",
     "emailBlurb": "Here's a document for you to sign",
     "templateId": "xxxx",
     "templateRoles": [
          {
              "email": "test@email.com",
              "name": "Test Person",
              "roleName": "parent_signer" }] }
  </code></pre>
  
  <p>When I try to add parameters for custom field filling, I get a 400 error:</p>
  
  <pre><code>{ "accountId": "xxx",
     "status": "sent",
     "emailSubject": "Please sign this document",
     "emailBlurb": "Here's a document for you to sign",
     "templateId": "xxxx",
     "templateRoles": [
          {
              "email": "test@email.com",
              "name": "Test Person",
              "tabs": [
                 { "textTabs":
                    [
                       {"tabLabel": "Doc_Name",
                        "name": "Doc_Name",
                        "value": "Doc Name Data Would Go Here"}
                    ]
                 }
              ],
              "roleName": "parent_signer" }] }
  </code></pre>
  
  <p>And the single document in my template in question has custom fields with those names. 
  <a href="https://imgur.com/z519zm3" rel="nofollow noreferrer">https://imgur.com/z519zm3</a></p>