Plugin Name: [Forminator] - Auto populate fields with post id.

forminator-populate-select-field-with-postid.php

<?php
/**
 * Plugin Name: [Forminator] - Auto populate fields with post id.
 * Plugin URI: https://premium.wpmudev.org/
 * Description: Autopopulate Forminator select field with current post id.
 * Author: Panos Lyrakis @ WPMUDEV
 * Author URI: https://premium.wpmudev.org/
 * License: GPLv2 or later
 */

if ( ! defined( 'ABSPATH' ) || ( defined( 'WP_CLI' ) && WP_CLI ) ) {
	return;
}

add_filter(
	'forminator_cform_render_fields',
	function( $wrappers, $model_id ) {
        if ( 6 !== $model_id ) { // Please change the form ID.
            return $wrappers;
        }

        $id = get_the_ID();
        if ( empty( $id ) ) {
            $id = sanitize_text_field( $_POST['extra']['page_id'] );
        }

		$select_field_id  = 'select-1';
		$selected_options = array(
			// post_id => options value
			15 => 'one',
			27 => 'two',
			63 => 'Option-3',
		);

		if ( in_array( $id, array_keys( $selected_options ) ) ) {
			$selected_value = $selected_options[ $id ];

			foreach ( $wrappers as $wrapper_key => $wrapper ) {
				if ( ! isset( $wrapper['fields'] ) ) {
					continue;
				}

				foreach ( $wrapper['fields'] as $wrapped_field_key => $wrapped_field ) {
					$field_id = $wrapper['fields'][ $wrapped_field_key ]['element_id'];

					if ( $select_field_id === $field_id && ! empty( $wrappers[ $wrapper_key ]['fields'][ $wrapped_field_key ]['options'] ) ) {
						foreach( $wrappers[ $wrapper_key ]['fields'][ $wrapped_field_key ]['options'] as $option_key => $option ) {
							if ( $selected_value === $wrappers[ $wrapper_key ]['fields'][ $wrapped_field_key ]['options'][ $option_key ]['value'] ) {
								$wrappers[ $wrapper_key ]['fields'][ $wrapped_field_key ]['options'][ $option_key ]['default'] = true;
							} else {
								$wrappers[ $wrapper_key ]['fields'][ $wrapped_field_key ]['options'][ $option_key ]['default'] = false;
							}
						}
					}

				}
			}
		}

		return $wrappers;
	},
	10,
	2
);

It seems not working on 1.26.