API contact custom field
good to hear you had some success! I would expect the api explorer to show you the custom fields for contact? You can use the custom fields with the api if you know the id of  the custom field. So for custom field with id 5 the parameter for the API would be:

 custom_5 =&gt; &lt;your value&gt;


So an example would be:

$result = civicrm_api3('Contact', 'create', [
  'custom_11' =&gt; "blablabla",
])


;