WP Simple Pay: Programmatically Set Custom Field Default Value

/**

 * @link https://library.wpcode.com/snippet/j57zmp2g/

 */

add_filter(

	// Update `123` to match your Form ID.

	// Update `2`to match the custom field ID.

	'simpay_form_123_field_2_default_value',

	/**

	 * @var string $default

	 * @return string

	 */

	function( $default ) {

		// Set the default value to the value of the current page's `_my_custom_field` metadata value.

		return get_post_meta( get_the_ID(), '_my_custom_field', true );

	}

);