Kite

Due to security/privacy concerns, JSON REST API (WP API) only allows post meta data (custom field value) for authenticated users. 
This plugin is an easy way to make post meta available when unauthenticated.
After activate the plugin, go to WP API Customizer option page. 
Click the plus icon to add rows. 
Set JSON attributes and custom field names in your posts that you want to use when unauthenticated. 
The JSON REST API will provide the data as “post_meta” like the following:
<pre><code>"post_meta":{"your_attribute_a":"Custom Field A value","your_attribute_b":"Custom Field B value"}
</code></pre>

WP API Customizer

Make post meta data (custom field values) available for JSON REST API (WP API) when unauthenticated.

گناهکار

USAGE: 
use [cf] shortcode in your post content to show the custom field value without edit your theme files.
EXAMPLE: 
[cf]somekey[/cf] 
returns value of ‘somekey’ custom-filed.

Custom fields shortcode

Use custom fields as shortcode in post content without edit your theme files.

WP API Customizer vs Custom fields shortcode