What's the Google Tag Manager?
Google Tag Manager is a tag management system that allows you to set up and manage tags on your site without changing your website's code.
https://support.google.com/tagmanager/answer/6102821
Where can I find the container ID? https://support.google.com/tagmanager/answer/15107467
Typeform help article https://help.typeform.com/hc/en-us/articles/360029583611-Add-Google-Tag-Manager-to-a-form
Fillout the help article https://www.fillout.com/help/google-tag-manager
Please note that it is currently not possible to use Google Tag Manager if your form includes a Payment field.