This will allow you to retrieve all custom fields set in your Kartra account.
TypeParametersValues
POSTcmd*retrieve_custom_fields

* Required fields Here is an example:

'actions' => [
    '0' => [
        'cmd' => 'retrieve_custom_fields'
    ]
]

Here are the results returned by the request:

ParametersValues
custom_fieldsString (JSON encoded array containing the details of all the custom fields in your account, empty array if there are no custom fields)
 
JSON parametersValues
field_idinteger A unique identifier for the custom field.
field_identifierstring The identifier string set when the field was created
field_typestring The field type with possible values: 'input_field', 'text_area', 'drop_down', 'radio_button', 'checkbox'.
field_valuearray Objects containing the option id and the option value. This applies only for multiple selection field types like 'drop_down', 'radio_button' and 'checkbox'.
Example of the returned array:
"custom_fields": [
    {
        "field_id": "1",
        "field_identifier": "gender",
        "field_type": "radio_button",
	"field_value":[
	    {
	        "option_id":"1",
		"option_value":"male",
            },
	    {
	        "option_id":"2",
		"option_value":"female",
	    }
	]
    },
    {
        "field_id": "2",
        "field_identifier": "fruits",
        "field_type": "checkbox",
        "field_value":[
	    {
	        "option_id":"1",
	        "option_value":"apple",
	    },
	    {
	        "option_id":"2",
	        "option_value":"pear",
	    }
	]
    },
    {
        "field_id": "3",
        "field_identifier": "field_3",
        "field_type": "input_field"
    },
    {
        "field_id": "4",
        "field_identifier": "field_4",
        "field_type": "drop_down"
        "field_value":[
	    {
	        "option_id":"1",
	        "option_value":"male",
	    },
	    {
	        "option_id":"2",
	        "option_value":"female",
	    }
	]            
    },
    {
        "field_id": "5",
        "field_identifier": "field_5",
        "field_type": "text_area"
    }
]