Setting up remote ping to MailChimp

Hello. Very much enjoying Membership Premium version 3.5.1.6, and am impressed by how helpful support is.

I am trying to set up my remote ping for MailChimp. I have tried to use the tutorial. I have also tried using the responses on this thread: https://premium.wpmudev.org/forums/topic/pings-to-mailchimp

Here is what I've filled in:

Ping URL: http://us7.api.mailchimp.com/1.3/?method=listSubscribe

Ping Data:
apikey=redacted-us7
id=redacted
first_name=%userfirstname%
last_name=%userlastname%
email_address=%useremail%
double_optin=false
update_existing=true
send_welcome=false

I am having no luck - every time I test the ping, I get this response "{"error":"Method listsubscribe is not exported by this server","code":-90}"

What am I doing wrong?

Thanks so much!