functions.php

<?php

/**
 * Include Form Fields on Confirmation URL
 * @see https://www.billerickson.net/code/wpforms-pass-form-data-to-another-form/
 */
function be_wpforms_fields_on_confirmation( $url, $form_id, $fields, $form_data, $entry_id ) {

	$classes = explode( ' ', $form_data['settings']['form_class'] );
	if( ! in_array( 'fields-to-confirmation', $classes ) )
		return $url;

	$args = array();
	foreach( $fields as $field ) {
		if( !empty( $field['value'] ) )
			$args['wpforms-' . $form_id . '-field_' . $field['id'] ] = $field['value'];
	}
	
	return esc_url_raw( add_query_arg( $args, $url ) );
}
add_filter( 'wpforms_process_redirect_url', 'be_wpforms_fields_on_confirmation', 10, 5 );