How to redirect subscriber confirmation to Gatsby/Netlify from Heroku

Everything seems to be in order: webhooks, Gatsby plugins, Mailgun, etc. However, whenever a new user subscribes to my site, the confirmation email link takes them to the Heroku app that’s hosting my Ghost admin. If possible, I’d like the user to be redirected to my root domain so that they’re not exposed to the Heroku backend. I’ve marked the Heroku site as private so that the Gatsby setup is truly “headless”. The user hits a login screen, which is bad.

Does anyone know how to redirect subscriber confirmation to Gatsby/Netlify from Heroku? Thanks in advance! Diagnostic details below:

Got my answer at Codebulbs.