opt in form for subscriber on blog problems HELP

I have just begun two blogs and have chosen an opt in form for subscribers as a plug in. It is compatible with my version of wp. I have had two people subscribe but they do not get directed back to my ‘thank you ‘ page. I have an api key that goes to mad mimi, but they aren’t getting the lists.
What am I doing wrong? I must be missing a step.
Thank you
https://www.lettersfromred.com