Additional fee if checkbox is checked while registration in PMPro plugin

Hi

I am using “Paid Memberships Pro – Register Helper Add On” plugin and added code in functions.php file to create a custom checkbox. checkbox is added and showing in “Account Information” section on checkout page.
screenshot: https://prntscr.com/vl7orw

Now I want to charge one time extra fee from user. If checkbox is checked, $5 should be added in the billing amount.

I tried the below link, but now seem working, fields were not showing on checkout page
https://gist.github.com/andrewlimaza/7a0dcf8478afafb0ce7126ac70cdb667

Please guide me how this can be done.
waiting for your quick response.

Thanks