Hi! :)
I needed Mailchimp and Webform Mailchimp modules to allow me to choose a segment on each of my forms in order to add the subscriber to a different Tag, after the subscription succeeds.
First of all, you'll need a patch on the Mailchimp module: https://www.drupal.org/project/mailchimp/issues/3136481 ✨ Add Member to a Segment after subscription Active
After that, you can use the attached patch here.
Active
5.10
Code
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
Attached is a re-roll of this patch for 5.10
I also added a check to see if mailchimp_campaign is enabled before adding the new field and made the field a "webform_select_other" field in case you want to directly add a segment ID.