Source code for https://formidable-masterminds.com/integrate-qr-code-with-formidable/

frm_save_api_response.php

<?php

/* this will write the QR Code URL to the form entry */
add_action( 'frmapi_post_response', 'frm_save_api_response', 10, 3 );
function frm_save_api_response( $response, $entry, $form_action ) {
	/* convert the response to a PHP object */
	$results = json_decode($response['body']);

	/* get the results code */
	$code = $response['response']['code'];

	/* get the qr code url */
	$qr_url = $results->result->qr;

	/* get the form id */
	$form_id = $entry->form_id;

	/* get the right field for the form
   * this code is designed to run on two different forms
   */
	$field_id = ($form_id == '6' ) ? 125 : 123;

	if ( $code == 200 ) {
	    FrmProEntryMeta::update_single_field( array(
	      'entry_id' => $entry->id,
	      'field_id' => $field_id,
	      'value'    => $qr_url,
	    ));
    }
}

frm_save_api_response_with_kint.php

<?php

/* warning: do not run this code without the kint debugger installed
 * it will crash your site
 */

/* this will write the QR Code URL to the form entry */
add_action( 'frmapi_post_response', 'frm_save_api_response', 10, 3 );
function frm_save_api_response( $response, $entry, $form_action ) {
  
  d($response);
  d($entry);
  ddd($form_action);
  
	/* convert the response to a PHP object */
	$results = json_decode($response['body']);

	/* get the results code */
	$code = $response['response']['code'];

	/* get the qr code url */
	$qr_url = $results->result->qr;

	/* get the form id */
	$form_id = $entry->form_id;

	/* get the right field for the form
   * this code is designed to run on two different forms
   */
	$field_id = ($form_id == '6' ) ? 125 : 123;

	if ( $code == 200 ) {
	    FrmProEntryMeta::update_single_field( array(
	      'entry_id' => $entry->id,
	      'field_id' => $field_id,
	      'value'    => $qr_url,
	    ));
    }
}