Why did you unsubscribe? (or post-submit message)

Hi team, I'd love to give people who unsubscribe an option to let me know why -- a short survey with radio buttons, OR at the very least be able to add some custom text with a live link to the "unsubscribe confirmation" page -- that could point people to a short survey.

My relationships with my audience is important - and having a way to gather this data would be very valuable.

Thanks!