If you are using a sign-up checkbox in your Contact Form 7 forms you sometimes want to set a different Mailchimp audience 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="mailchimp-audience-id-here" />

Make sure to change mailchimp-audience-id-here into the actual ID of the Mailchimp audience you want people to subscribe to. You can find the ID of your Mailchimp audience by navigating to WP Admin > MC4WP > Mailchimp

This is particularly useful if you have an audience for each language and want to subscribe people to a certain audience based on which form (language) was being used.