Steps for get PRIVATE API KEY:

Step 1: Login to your Mailchimp Admin Portal. Mailchimp Portal

Step 2: Go to your Mailchimp account, click on profile and Click on Account & billing.
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Step 3: Then click on extras and choose API Keys from the drop-down menu.
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Step 4: Click to Create API Key to generate a new API Key.
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Step 5: Give name to API Key and click on Generate key.
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Step 6: Copy the Private Api Key
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Step 7: Paste in below text field

Step 8: Click on Verify button.