I was also struggling on this for a while because above "Accepeted Answer" is incorrect.The ID starting with CID_ is the "contactId", while the ID starting with CGC_ is the "contactLookupId". The contactLookupId will be generated, when you add a contact to a MailingList. The tricky part is the fact, that when you list all contacts in a MailingList via API https://api.qualtrics.com/api-reference/b3A6NjEwMTA-list-contacts-in-mailing-listthen you will only get the CID_ for each contact in that result list, but not the CGC_ info. To get that one you are forces to use the API getting the details of a single(!) contact in the MailingList (by adding the CID_ at the end of the API-URL):https://api.qualtrics.com/api-reference/b3A6NjEwMTQ-get-contact-in-mailing-listThat info includes the CGC_ id for the contact as well.Get contactLookupId.ps1.txtAttached is a sample Powershell code that shows the exact steps in detail.
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