Edit _Post custom field combine 2 field and add to a field

Hi, to combine values of 2 fields when saving, please use this snippet:

add_action( 'rwmb_after_save_post', function( $post_id ) {
    if ( isset( $_POST['field_1'] ) && isset( $_POST['field_2'] ) ) {
        $value = $_POST['field_1'] . $_POST['field_2'];
        update_post_meta( $post_id, 'field_combine', $value );
    }
} );

This snippet combines the values of field_1 and field_2 into field_combine.

To add JS to a meta box, please use this snippet:

add_action( 'rwmb_enqueue_scripts', function() {
    wp_enqueue_script( 'your-script', 'https://url-to-your-js-file', array( 'jquery' ), '', true );
} ):

List of actions are available in the documentation.

To add small HTML and JS just below a field, you can use the after param of the field. See field settings for more details.