Membership: Mailchimp auto-subscribe ping

Hiya,

As my first foray into pings, I am trying to auto-subscribe members to my mailchimp list, which I've done following Barry's instructions here:
https://premium.wpmudev.org/forums/topic/documentation-on-ping#post-122969

The only thing that I couldn't set is the option to "Strip query strings from URL" (not sure why but I don't seem to have that on my ping set-up dashboard) - see screenshot.

The pings history shows a 200-OK status, but my API dashboard on mailchimp reports that no API calls are being made (it seems that it would list failed attempts if there were any). I've tried setting the double-optin to false (else mailchimp doesn't add subscribers until they have manually opted in), tried get and post methods... not sure where to look next: any advice appreciated.

Thanks, Julie