Add meta data for a CPT to the REST response. `wp-json/wp/v2/poststatus_partners?_embed`

register_rest_fields.php

<?php

add_action( 'rest_api_init', function() {
	register_rest_field(
		array( 'poststatus_partners' ),
		'partner_meta',
		array(
			'get_callback'    => 'poststatus_get_restable_field_values_for_partner_meta',
			'update_callback' => 'poststatus_update_restable_field_values_for_partner_meta',
			'schema' => null,
		)
	);
} );

/**
 * Handler for getting custom field data.
 * @since  2.2.0
 * @param  array           $object   The object from the response
 * @param  string          $field_id Name of field
 * @param  WP_REST_Request $request  Current request
 * @return mixed
 */
function poststatus_get_restable_field_values_for_partner_meta( $object, $field_id, $request ) {

	return array(
		'button_text'     => get_post_meta( $object['id'], 'ps_partner_button_text', true ),
		'button_color'    => get_post_meta( $object['id'], 'ps_partner_button_color', true ),
		'destination_url' => get_post_meta( $object['id'], 'ps_partner_url', true ),
		'partner_label'   => get_post_meta( $object['id'], 'ps_partner_label', true ),
	);
}

/**
 * Handler for updating custom field data.
 * @since  2.2.0
 * @param  mixed    $value    The value of the field
 * @param  object   $object   The object from the response
 * @param  string   $field_id Name of field
 * @return bool|int
 */
function poststatus_update_restable_field_values_for_partner_meta( $values, $object, $field_id ) {
	if ( empty( $values ) || ! is_array( $values ) || 'partner_meta' !== $field_id ) {
		return;
	}

	// Do update to values and return the updated values.

	return array(
		'button_text'     => $updated['button_text'],
		'button_color'    => $updated['button_color'],
		'destination_url' => $updated['destination_url'],
		'partner_label'   => $updated['partner_label']
	);

	return $updated;
}