I have this plugin called Optin Revolution. It's a plugin where you can create a popup for people to subscribe to something. And you can use different sites/accounts for the email to creat a list on. ONe of them is MailChimp. I selected that. But there's something they are asking that I don't know what it is..
That's one part of the settings. What is the 'U'?? I found the ID in the list options. And when I test it out without the 'U' filled, i get this: http://google.us5.list-manage1.com/subscribe/post
Can anyone help with this?