Add Dynamic Hidden Field

/**

 * In this example, a new donation field is created but left out of the

 * front-end donation form. Instead, it is dynamically set as a hidden

 * field in the donation form, based on the ID of the page that the

 * donation form is on.

 *

 * Here, the profile ID is only recorded when the donation is made on a

 * custom post type with a post type of "profile".

 */

add_action(

	'init',

	function() {

		/* Create the Donation Field instance. */

		$field = new Charitable_Donation_Field(

			'profile_id',

			array(

				'label'          => 'Profile ID',

				'data_type'      => 'meta',

				'value_callback' => false,

				'donation_form'  => false,

				'admin_form'     => true,

				'show_in_meta'   => true,

				'show_in_export' => true,

				'email_tag'      => false,

			)

		);

		/* Register it. */

		charitable()->donation_fields()->register_field( $field );

	}

);

add_filter(

	'charitable_donation_form_hidden_fields',

	function( $fields ) {

		/* Only record a "profile_id" when the current post type is "profile". */

		if ( 'profile' == get_post_type() ) {

			$fields['profile_id'] = get_the_ID();

		}

		return $fields;

	}

);