If you are using a sign-up checkbox in your Contact Form 7 forms you sometimes want to set a different Mailchimp list depending on which form is being used. To do this, you can include the following HTML in your CF7 forms which defines the list the plugin will subscribe the person to.
<input name="_mc4wp_lists" type="hidden" value="123abc456def" />
Make sure to change
123abc456def into the actual ID of the Mailchimp list you want people to subscribe to. You can find the ID of your Mailchimp lists by navigating to WP Admin > MC4WP > Mailchimp
This is particularly useful if you have a list for each language and want to subscribe people to a certain list based on which form (language) was being used.